Chapter 9 The tactic language
This chapter gives a compact documentation of Ltac, the tactic language available in Coq. We start by giving the syntax, and next, we present the informal semantics. If you want to know more regarding this language and especially about its foundations, you can refer to [43]. Chapter 10 is devoted to giving examples of use of this language on small but also with nontrivial problems.
9.1 Syntax
The syntax of the tactic language is given Figures 9.1 and 9.2. See Chapter 1 for a description of the BNF metasyntax used in these grammar rules. Various already defined entries will be used in this chapter: entries natural, integer, ident, qualid, term, cpattern and atomic_tactic represent respectively the natural and integer numbers, the authorized identificators and qualified names, Coq’s terms and patterns and all the atomic tactics described in Chapter 8. The syntax of cpattern is the same as that of terms, but it is extended with pattern matching metavariables. In cpattern, a patternmatching metavariable is represented with the syntax ?id where id is an ident. The notation _ can also be used to denote metavariable whose instance is irrelevant. In the notation ?id, the identifier allows us to keep instantiations and to make constraints whereas _ shows that we are not interested in what will be matched. On the right hand side of patternmatching clauses, the named metavariable are used without the question mark prefix. There is also a special notation for secondorder patternmatching problems: in an applicative pattern of the form @?id id_{1} …id_{n}, the variable id matches any complex expression with (possible) dependencies in the variables id_{1} …id_{n} and returns a functional term of the form fun id_{1} …id_{n} => term.
The main entry of the grammar is expr. This language is used in proof mode but it can also be used in toplevel definitions as shown in Figure 9.3.
Remarks:
 The infix tacticals “…  …”, “… + …”, and “… ; …” are associative.
 In tacarg, there is an overlap between qualid as a direct tactic argument and qualid as a particular case of term. The resolution is done by first looking for a reference of the tactic language and if it fails, for a reference to a term. To force the resolution as a reference of the tactic language, use the form ltac : qualid. To force the resolution as a reference to a term, use the syntax (qualid).
 As shown by the figure, tactical  binds more than the
prefix tacticals try, repeat, do and
abstract which themselves bind more than the postfix tactical
“… ;[ … ]” which binds more than “… ;
…”.
For instance
try repeat tactic_{1}  tactic_{2};tactic_{3};[tactic_{31}…tactic_{3n}];tactic_{4}.
is understood as
(try (repeat (tactic_{1}  tactic_{2})));
((tactic_{3};[tactic_{31}…tactic_{3n}]);tactic_{4}).
expr ::= expr ; expr  [> expr  …  expr ]  expr ; [ expr  …  expr ]  tacexpr_{3} tacexpr_{3} ::= do (natural  ident) tacexpr_{3}  progress tacexpr_{3}  repeat tacexpr_{3}  try tacexpr_{3}  once tacexpr_{3}  exactly_once tacexpr_{3}  timeout (natural  ident) tacexpr_{3}  time [string] tacexpr_{3}  tacexpr_{2} tacexpr_{2} ::= tacexpr_{1}  tacexpr_{3}  tacexpr_{1} + tacexpr_{3}  tryif tacexpr_{1} then tacexpr_{1} else tacexpr_{1}  tacexpr_{1} tacexpr_{1} ::= fun name … name => atom  let [rec] let_clause with … with let_clause in atom  match goal with context_rule  …  context_rule end  match reverse goal with context_rule  …  context_rule end  match expr with match_rule  …  match_rule end  lazymatch goal with context_rule  …  context_rule end  lazymatch reverse goal with context_rule  …  context_rule end  lazymatch expr with match_rule  …  match_rule end  multimatch goal with context_rule  …  context_rule end  multimatch reverse goal with context_rule  …  context_rule end  multimatch expr with match_rule  …  match_rule end  abstract atom  abstract atom using ident  first [ expr  …  expr ]  solve [ expr  …  expr ]  idtac [message_token … message_token]  fail [natural] [message_token … message_token]  gfail [natural] [message_token … message_token]  fresh  fresh string fresh qualid  context ident [ term ]  eval redexpr in term  type of term  external string string tacarg … tacarg  constr : term  uconstr : term  type_term term  numgoals  guard test  atomic_tactic  qualid tacarg … tacarg  atom
atom ::= qualid  ()  integer  ( expr ) message_token ::= string  ident  integer tacarg ::= qualid  ()  ltac : atom  term let_clause ::= ident [name … name] := expr context_rule ::= context_hyp , … , context_hyp cpattern => expr   cpattern => expr  _ => expr context_hyp ::= name : cpattern  name := cpattern [: cpattern] match_rule ::= cpattern => expr  context [ident] [ cpattern ] => expr  appcontext [ident] [ cpattern ] => expr  _ => expr test ::= integer = integer  integer < integer  integer <= integer  integer > integer  integer >= integer
top ::= [Local] Ltac ltac_def with … with ltac_def ltac_def ::= ident [ident … ident] := expr  qualid [ident … ident] ::=expr
9.2 Semantics
Tactic expressions can only be applied in the context of a proof. The evaluation yields either a term, an integer or a tactic. Intermediary results can be terms or integers but the final result must be a tactic which is then applied to the focused goals.
There is a special case for match goal expressions of which the clauses evaluate to tactics. Such expressions can only be used as end result of a tactic expression (never as argument of a non recursive local definition or of an application).
The rest of this section explains the semantics of every construction of Ltac.
Sequence
A sequence is an expression of the following form:
expr_{1} ; expr_{2}
The expressions expr_{1} and expr_{2} are evaluated to v_{1} and v_{2} which have to be tactic values. The tactic v_{1} is then applied and v_{2} is applied to the goals generated by the application of v_{1}. Sequence is leftassociative.
Local application of tactics
Different tactics can be applied to the different goals using the following form:
[ > expr_{1}  ...  expr_{n} ]
The expressions expr_{i} are evaluated to v_{i}, for i=0,...,n and all have to be tactics. The v_{i} is applied to the ith goal, for =1,...,n. It fails if the number of focused goals is not exactly n.
Variants:
 If no tactic is given for the ith goal, it behaves as if the tactic idtac were given. For instance, [ >  auto ] is a shortcut for [ > idtac  auto ].
 [ > expr_{1}  ... 
expr_{i}  expr .. 
expr_{i+1+j}  ...  expr_{n} ]
In this variant, expr is used for each goal numbered from i+1 to i+j (assuming n is the number of goals).
Note that .. is part of the syntax, while ... is the metasymbol used to describe a list of expr of arbitrary length. goals numbered from i+1 to i+j.
 [ > expr_{1}  ... 
expr_{i}  ..  expr_{i+1+j} 
...  expr_{n} ]
In this variant, idtac is used for the goals numbered from i+1 to i+j.
 [ > expr .. ]
In this variant, the tactic expr is applied independently to each of the goals, rather than globally. In particular, if there are no goal, the tactic is not run at all. A tactic which expects multiple goals, such as swap, would act as if a single goal is focused.
 expr ; [ expr_{1}  ...  expr_{n} ]
This variant of local tactic application is paired with a sequence. In this variant, n must be the number of goals generated by the application of expr to each of the individual goals independently. All the above variants work in this form too. Formally, expr ; [ ... ] is equivalent to
[ > expr ; [ > ... ] .. ]
For loop
There is a for loop that repeats a tactic num times:
do num expr
expr is evaluated to v which must be a tactic value. This tactic value v is applied num times. Supposing num>1, after the first application of v, v is applied, at least once, to the generated subgoals and so on. It fails if the application of v fails before the num applications have been completed.
Repeat loop
We have a repeat loop with:
repeat expr
expr is evaluated to v. If v denotes a tactic, this tactic is applied to each focused goal independently. If the application succeeds, the tactic is applied recursively to all the generated subgoals until it eventually fails. The recursion stops in a subgoal when the tactic has failed to make progress. The tactic repeat expr itself never fails.
Error catching
We can catch the tactic errors with:
try expr
expr is evaluated to v which must be a tactic value. The tactic value v is applied to each focused goal independently. If the application of v fails in a goal, it catches the error and leaves the goal unchanged. If the level of the exception is positive, then the exception is reraised with its level decremented.
Detecting progress
We can check if a tactic made progress with:
progress expr
expr is evaluated to v which must be a tactic value. The tactic value v is applied to each focued subgoal independently. If the application of v to one of the focused subgoal produced subgoals equal to the initial goals (up to syntactical equality), then an error of level 0 is raised.
Error message: Failed to progress
Backtracking branching
We can branch with the following structure:
expr_{1} + expr_{2}
expr_{1} and expr_{2} are evaluated to v_{1} and v_{2} which must be tactic values. The tactic value v_{1} is applied to each focused goal independently and if it fails or a later tactic fails, then the proof backtracks to the current goal and v_{2} is applied.
Tactics can be seen as having several successes. When a tactic fails it asks for more successes of the prior tactics. expr_{1} + expr_{2} has all the successes of v_{1} followed by all the successes of v_{2}. Algebraically, (expr_{1} + expr_{2});expr_{3} = (expr_{1};expr_{3}) + (expr_{2};expr_{3}).
Branching is leftassociative.
First tactic to work
Backtracking branching may be too expensive. In this case we may restrict to a local, left biased, branching and consider the first tactic to work (i.e. which does not fail) among a panel of tactics:
first [ expr_{1}  ...  expr_{n} ]
expr_{i} are evaluated to v_{i} and v_{i} must be tactic values, for i=1,...,n. Supposing n>1, it applies, in each focused goal independently, v_{1}, if it works, it stops otherwise it tries to apply v_{2} and so on. It fails when there is no applicable tactic. In other words, first [ expr_{1}  ...  expr_{n} ] behaves, in each goal, as the the first v_{i} to have at least one success.
Error message: No applicable tactic
Leftbiased branching
Yet another way of branching without backtracking is the following structure:
expr_{1}  expr_{2}
expr_{1} and expr_{2} are evaluated to v_{1} and v_{2} which must be tactic values. The tactic value v_{1} is applied in each subgoal independently and if it fails to progress then v_{2} is applied. expr_{1}  expr_{2} is equivalent to first [ progress expr_{1}  progress expr_{2} ] (except that if it fails, it fails like v_{2}). Branching is leftassociative.
Generalized biased branching
The tactic
tryif expr_{1} then expr_{2} else expr_{3}
is a generalization of the biasedbranching tactics above. The expression expr_{1} is evaluated to v_{1}, which is then applied to each subgoal independently. For each goal where v_{1} succeeds at least once, tacexpr_{2} is evaluated to v_{2} which is then applied collectively to the generated subgoals. The v_{2} tactic can trigger backtracking points in v_{1}: where v_{1} succeeds at least once, tryif expr_{1} then expr_{2} else expr_{3} is equivalent to v_{1};v_{2}. In each of the goals where v_{1} does not succeed at least once, expr_{3} is evaluated in v_{3} which is is then applied to the goal.
Soft cut
Another way of restricting backtracking is to restrict a tactic to a single success a posteriori:
once expr
expr is evaluated to v which must be a tactic value. The tactic value v is applied but only its first success is used. If v fails, once expr fails like v. If v has a least one success, once expr succeeds once, but cannot produce more successes.
Checking the successes
Coq provides an experimental way to check that a tactic has exactly one success:
exactly_once expr
expr is evaluated to v which must be a tactic value. The tactic value v is applied if it has at most one success. If v fails, exactly_once expr fails like v. If v has a exactly one success, exactly_once expr succeeds like v. If v has two or more successes, exactly_once expr fails.
The experimental status of this tactic pertains to the fact if v performs side effects, they may occur in a unpredictable way. Indeed, normally v would only be executed up to the first success until backtracking is needed, however exactly_once needs to look ahead to see whether a second success exists, and may run further effects immediately.
Error message: This tactic has more than one success
Solving
We may consider the first to solve (i.e. which generates no subgoal) among a panel of tactics:
solve [ expr_{1}  ...  expr_{n} ]
expr_{i} are evaluated to v_{i} and v_{i} must be tactic values, for i=1,...,n. Supposing n>1, it applies v_{1} to each goal independently, if it doesn’t solve the goal then it tries to apply v_{2} and so on. It fails if there is no solving tactic.
Error message: Cannot solve the goal
Identity
The constant idtac is the identity tactic: it leaves any goal unchanged but it appears in the proof script.
Variant: idtac message_token … message_token
This prints the given tokens. Strings and integers are printed literally. If a (term) variable is given, its contents are printed.
Failing
The tactic fail is the alwaysfailing tactic: it does not solve any goal. It is useful for defining other tacticals since it can be caught by try, repeat, match goal, or the branching tacticals. The fail tactic will, however, succeed if all the goals have already been solved.
Variants:

fail n
The number n is the failure level. If no level is specified, it defaults to 0. The level is used by try, repeat, match goal and the branching tacticals. If 0, it makes match goal considering the next clause (backtracking). If non zero, the current match goal block, try, repeat, or branching command is aborted and the level is decremented. In the case of +, a nonzero level skips the first backtrack point, even if the call to fail n is not enclosed in a + command, respecting the algebraic identity.  fail message_token … message_token
The given tokens are used for printing the failure message.  fail n message_token … message_token
This is a combination of the previous variants.  gfail
This variant fails even if there are no goals left.  gfail message_token … message_token
gfail n message_token … message_token
These variants fail with an error message or an error level even if there are no goals left. Be careful however if Coq terms have to be printed as part of the failure: term construction always forces the tactic into the goals, meaning that if there are no goals when it is evaluated, a tactic call like let x:=H in fail 0 x will succeed.
Error message: Tactic Failure message (level n).
Timeout
We can force a tactic to stop if it has not finished after a certain amount of time:
timeout num expr
expr is evaluated to v which must be a tactic value. The tactic value v is applied normally, except that it is interrupted after num seconds if it is still running. In this case the outcome is a failure.
Warning: For the moment, timeout is based on elapsed time in seconds, which is very machinedependent: a script that works on a quick machine may fail on a slow one. The converse is even possible if you combine a timeout with some other tacticals. This tactical is hence proposed only for convenience during debug or other development phases, we strongly advise you to not leave any timeout in final scripts. Note also that this tactical isn’t available on the native Windows port of Coq.
Timing a tactic
A tactic execution can be timed:
time string expr
evaluates expr and displays the time the tactic expression ran, whether it fails or successes. In case of several successes, the time for each successive runs is displayed. Time is in seconds and is machinedependent. The string argument is optional. When provided, it is used to identify this particular occurrence of time.
Local definitions
Local definitions can be done as follows:
let ident_{1} := expr_{1}
with ident_{2} := expr_{2}
...
with ident_{n} := expr_{n} in
expr
each expr_{i} is evaluated to v_{i}, then, expr is evaluated by substituting v_{i} to each occurrence of ident_{i}, for i=1,...,n. There is no dependencies between the expr_{i} and the ident_{i}.
Local definitions can be recursive by using let rec instead of let. In this latter case, the definitions are evaluated lazily so that the rec keyword can be used also in non recursive cases so as to avoid the eager evaluation of local definitions.
Application
An application is an expression of the following form:
qualid tacarg_{1} ... tacarg_{n}
The reference qualid must be bound to some defined tactic definition expecting at least n arguments. The expressions expr_{i} are evaluated to v_{i}, for i=1,...,n.
Function construction
A parameterized tactic can be built anonymously (without resorting to local definitions) with:
fun ident_{1} ... ident_{n} => expr
Indeed, local definitions of functions are a syntactic sugar for binding a fun tactic to an identifier.
Pattern matching on terms
We can carry out pattern matching on terms with:
match expr with
cpattern_{1} => expr_{1}
 cpattern_{2} => expr_{2}
...
 cpattern_{n} => expr_{n}
 _ => expr_{n+1}
end
The expression expr is evaluated and should yield a term which is matched against cpattern_{1}. The matching is nonlinear: if a metavariable occurs more than once, it should match the same expression every time. It is firstorder except on the variables of the form @?id that occur in head position of an application. For these variables, the matching is secondorder and returns a functional term.
Alternatively, when a metavariable of the form ?id occurs under binders, say x_{1}, …, x_{n} and the expression matches, the metavariable is instantiated by a term which can then be used in any context which also binds the variables x_{1}, …, x_{n} with same types. This provides with a primitive form of matching under context which does not require manipulating a functional term.
If the matching with cpattern_{1} succeeds, then expr_{1} is evaluated into some value by substituting the pattern matching instantiations to the metavariables. If expr_{1} evaluates to a tactic and the match expression is in position to be applied to a goal (e.g. it is not bound to a variable by a let in), then this tactic is applied. If the tactic succeeds, the list of resulting subgoals is the result of the match expression. If expr_{1} does not evaluate to a tactic or if the match expression is not in position to be applied to a goal, then the result of the evaluation of expr_{1} is the result of the match expression.
If the matching with cpattern_{1} fails, or if it succeeds but the evaluation of expr_{1} fails, or if the evaluation of expr_{1} succeeds but returns a tactic in execution position whose execution fails, then cpattern_{2} is used and so on. The pattern _ matches any term and shunts all remaining patterns if any. If all clauses fail (in particular, there is no pattern _) then a nomatchingclause error is raised.
Failures in subsequent tactics do not cause backtracking to select new branches or inside the righthand side of the selected branch even if it has backtracking points.
Error messages:
 No matching clauses for match
No pattern can be used and, in particular, there is no _ pattern.
 Argument of match does not evaluate to a term
This happens when expr does not denote a term.
Variants:

Using multimatch instead of match will allow subsequent
tactics to backtrack into a righthand side tactic which has
backtracking points left and trigger the selection of a new matching
branch when all the backtracking points of the righthand side have
been consumed.
The syntax match … is, in fact, a shorthand for once multimatch ….
 Using lazymatch instead of match will perform the same pattern matching procedure but will commit to the first matching branch rather than trying a new matching if the righthand side fails. If the righthand side of the selected branch is a tactic with backtracking points, then subsequent failures cause this tactic to backtrack.

There is a special form of patterns to match a subterm against the
pattern:
context ident [ cpattern ]
It matches any term with a subterm matching cpattern. If there is a match, the optional ident is assigned the “matched context”, i.e. the initial term where the matched subterm is replaced by a hole. The example below will show how to use such term contexts.If the evaluation of the righthandside of a valid match fails, the next matching subterm is tried. If no further subterm matches, the next clause is tried. Matching subterms are considered topbottom and from left to right (with respect to the raw printing obtained by setting option Printing All, see Section 2.9).
Coq < Ltac f x :=
match x with
context f [S ?X] =>
idtac X; (* To display the evaluation order *)
assert (p := eq_refl 1 : X=1); (* To filter the case X=1 *)
let x:= context f[O] in assert (x=O) (* To observe the context *)
end.
f is defined
Coq < Goal True.
1 subgoal
============================
True
Coq < f (3+4).
2
1
2 subgoals
p : 1 = 1
============================
1 + 4 = 0
subgoal 2 is:
True

For historical reasons, context used to consider nary applications
such as (f 1 2) as a whole, and not as a sequence of unary
applications ((f 1) 2). Hence context [f ?x] would fail
to find a matching subterm in (f 1 2): if the pattern was a partial
application, the matched subterms would have necessarily been
applications with exactly the same number of arguments.
As a workaround, one could use the following variant of context:
appcontext ident [ cpattern ]
This syntax is now deprecated, as context behaves as intended. The former behavior can be retrieved with the Tactic Compat Context flag.
Pattern matching on goals
We can make pattern matching on goals using the following expression:
match goal with  hyp_{1,1},...,hyp_{1,m1} cpattern_{1}=> expr_{1}  hyp_{2,1},...,hyp_{2,m2} cpattern_{2}=> expr_{2} ...  hyp_{n,1},...,hyp_{n,mn} cpattern_{n}=> expr_{n} _ => expr_{n+1} end
If each hypothesis pattern hyp_{1,i}, with i=1,...,m_{1} is matched (nonlinear firstorder unification) by an hypothesis of the goal and if cpattern_{1} is matched by the conclusion of the goal, then expr_{1} is evaluated to v_{1} by substituting the pattern matching to the metavariables and the real hypothesis names bound to the possible hypothesis names occurring in the hypothesis patterns. If v_{1} is a tactic value, then it is applied to the goal. If this application fails, then another combination of hypotheses is tried with the same proof context pattern. If there is no other combination of hypotheses then the second proof context pattern is tried and so on. If the next to last proof context pattern fails then expr_{n+1} is evaluated to v_{n+1} and v_{n+1} is applied. Note also that matching against subterms (using the context ident [ cpattern ]) is available and is also subject to yielding several matchings.
Failures in subsequent tactics do not cause backtracking to select new branches or combinations of hypotheses, or inside the righthand side of the selected branch even if it has backtracking points.
Error message: No matching clauses for match goal
No clause succeeds, i.e. all matching patterns, if any, fail at the application of the righthandside.
It is important to know that each hypothesis of the goal can be matched by at most one hypothesis pattern. The order of matching is the following: hypothesis patterns are examined from the right to the left (i.e. hyp_{i,mi} before hyp_{i,1}). For each hypothesis pattern, the goal hypothesis are matched in order (fresher hypothesis first), but it possible to reverse this order (older first) with the match reverse goal with variant.
Variant:
Using multimatch instead of match will allow subsequent tactics to backtrack into a righthand side tactic which has backtracking points left and trigger the selection of a new matching branch or combination of hypotheses when all the backtracking points of the righthand side have been consumed.
The syntax match [reverse] goal … is, in fact, a shorthand for once multimatch [reverse] goal ….
Using lazymatch instead of match will perform the same pattern matching procedure but will commit to the first matching branch with the first matching combination of hypotheses rather than trying a new matching if the righthand side fails. If the righthand side of the selected branch is a tactic with backtracking points, then subsequent failures cause this tactic to backtrack.
Filling a term context
The following expression is not a tactic in the sense that it does not produce subgoals but generates a term to be used in tactic expressions:
context ident [ expr ]
ident must denote a context variable bound by a context pattern of a match expression. This expression evaluates replaces the hole of the value of ident by the value of expr.
Error message: not a context variable
Generating fresh hypothesis names
Tactics sometimes have to generate new names for hypothesis. Letting the system decide a name with the intro tactic is not so good since it is very awkward to retrieve the name the system gave. The following expression returns an identifier:
fresh component … component
It evaluates to an identifier unbound in the goal. This fresh identifier is obtained by concatenating the value of the component’s (each of them is, either an qualid which has to refer to a (unqualified) name, or directly a name denoted by a string). If the resulting name is already used, it is padded with a number so that it becomes fresh. If no component is given, the name is a fresh derivative of the name H.
Computing in a constr
Evaluation of a term can be performed with:
eval redexpr in term
where redexpr is a reduction tactic among red, hnf, compute, simpl, cbv, lazy, unfold, fold, pattern.
Recovering the type of a term
The following returns the type of term:
type of term
Manipulating untyped terms
The terms built in Ltac are welltyped by default. It may not be appropriate for building large terms using a recursive Ltac function: the term has to be entirely type checked at each step, resulting in potentially very slow behavior. It is possible to build untyped terms using Ltac with the syntax
uconstr : term
An untyped term, in Ltac, can contain references to hypotheses or to Ltac variables containing typed or untyped terms. An untyped term can be typechecked using the function type_term whose argument is parsed as an untyped term and returns a welltyped term which can be used in tactics.
type_term term
Untyped terms built using uconstr : can also be used as arguments to the refine tactic 8.2.3. In that case the untyped term is type checked against the conclusion of the goal, and the holes which are not solved by the typing procedure are turned into new subgoals.
Counting the goals
The number of goals under focus can be recovered using the numgoals function. Combined with the guard command below, it can be used to branch over the number of goals produced by previous tactics.
Coq < Goal True /\ True /\ True.
Coq < split;[split].
Coq < all:pr_numgoals.
There are 3 goals
3 subgoals
============================
True
subgoal 2 is:
True
subgoal 3 is:
True
Testing boolean expressions
The guard tactic tests a boolean expression, and fails if the expression evaluates to false. If the expression evaluates to true, it succeeds without affecting the proof.
guard test
The accepted tests are simple integer comparisons.
Coq < split;[split].
Coq < all:let n:= numgoals in guard n<4.
3 subgoals
============================
True
subgoal 2 is:
True
subgoal 3 is:
True
Coq < Fail all:let n:= numgoals in guard n=2.
The command has indeed failed with message:
Error: Condition not satisfied: 3=2
3 subgoals
============================
True
subgoal 2 is:
True
subgoal 3 is:
True
Error messages:
Proving a subgoal as a separate lemma
From the outside “abstract expr” is the same as solve expr. Internally it saves an auxiliary lemma called ident_subproofn where ident is the name of the current goal and n is chosen so that this is a fresh name. Such auxiliary lemma is inlined in the final proof term unless the proof is ended with “Qed exporting”. In such case the lemma is preserved. The syntax “Qed exporting ident_{1}, ..., ident_{n}” is also supported. In such case the system checks that the names given by the user actually exist when the proof is ended.
This tactical is useful with tactics such as omega or discriminate that generate huge proof terms. With that tool the user can avoid the explosion at time of the Save command without having to cut manually the proof in smaller lemmas.
Variants:

abstract expr using ident.
Give explicitly the name of the auxiliary lemma.
Error message: Proof is not complete
9.3 Tactic toplevel definitions
9.3.1 Defining L_{tac} functions
Basically, L_{tac} toplevel definitions are made as follows:
Ltac ident ident_{1} ... ident_{n} := expr
This defines a new L_{tac} function that can be used in any tactic script or new L_{tac} toplevel definition.
Remark: The preceding definition can equivalently be written:
Ltac ident := fun ident_{1} ... ident_{n} => expr
Recursive and mutual recursive function definitions are also possible with the syntax:
Ltac ident_{1} ident_{1,1} ... ident_{1,m1} := expr_{1}
with ident_{2} ident_{2,1} ... ident_{2,m2} := expr_{2}
...
with ident_{n} ident_{n,1} ... ident_{n,mn} := expr_{n}
It is also possible to redefine an existing userdefined tactic
using the syntax:
Ltac qualid ident_{1} ... ident_{n} ::= expr
A previous definition of qualidmust exist in the environment. The new definition will always be used instead of the old one and it goes accross module boundaries.
If preceded by the keyword Local the tactic definition will not be exported outside the current module.
9.3.2 Printing L_{tac} tactics
Defined L_{tac} functions can be displayed using the command
Print Ltac qualid.
9.4 Debugging L_{tac} tactics
9.4.1 Info trace
It is possible to print the trace of the path eventually taken by an L_{tac} script. That is, the list of executed tactics, discarding all the branches which have failed. To that end the Info command can be used with the following syntax.
Info num expr.
The number num is the unfolding level of tactics in the trace. At level 0, the trace contains a sequence of tactics in the actual script, at level 1, the trace will be the concatenation of the traces of these tactics, etc…
Coq < Goal exists n, n=0.
Coq < Info 0 t 1t 0.
t 0
No more subgoals.
Coq < Undo.
Coq < Info 1 t 1t 0.
exists 0;reflexivity
No more subgoals.
The trace produced by Info tries its best to be a reparsable L_{tac} script, but this goal is not achievable in all generality. So some of the output traces will contain oddities.
As an additional help for debugging, the trace produced by Info contains (in comments) the messages produced by the idtac tacticals 9.2 at the right possition in the script. In particular, the calls to idtac in branches which failed are not printed.
An alternative to the Info command is to use the Info Level option as follows:
Set Info Level num.
This will automatically print the same trace as Info num at each tactic call. The unfolding level can be overridden by a call to the Info command. And this option can be turned off with:
Unset Info Level num.
The current value for the Info Level option can be checked using the Test Info Level command.
9.4.2 Interactive debugger
The L_{tac} interpreter comes with a stepbystep debugger. The debugger can be activated using the command
Set Ltac Debug.
and deactivated using the command
Unset Ltac Debug.
To know if the debugger is on, use the command Test Ltac Debug. When the debugger is activated, it stops at every step of the evaluation of the current L_{tac} expression and it prints information on what it is doing. The debugger stops, prompting for a command which can be one of the following:
simple newline:  go to the next step 
h:  get help 
x:  exit current evaluation 
s:  continue current evaluation without stopping 
r n:  advance n steps further 
r string:  advance up to the next call to “idtac string” 