CHOOSE_TAC : thm_tactic
STRUCTURE
Tactic
SYNOPSIS
Adds the body of an existentially quantified theorem to the assumptions of a goal.
DESCRIPTION
When applied to a theorem A' |- ?x. t and a goal, CHOOSE_TAC adds t[x'/x] to the assumptions of the goal, where x' is a variant of x which is not free in the goal or assumption list; normally x' is just x. 
         A ?- u
   ====================  CHOOSE_TAC (A' |- ?x. t)
    A u {t[x'/x]} ?- u
Unless A' is a subset of A, this is not a valid tactic.
FAILURE
Fails unless the given theorem is existentially quantified.
EXAMPLE
Suppose we have a goal asserting that the output of an electrical circuit (represented as a boolean-valued function) will become high at some time: 
   ?- ?t. output(t)
and we have the following theorems available: 
   t1 = |- ?t. input(t)
   t2 = !t. input(t) ==> output(t+1)
Then the goal can be solved by the application of: 
   CHOOSE_TAC th1
     THEN EXISTS_TAC (Term `t+1`)
     THEN UNDISCH_TAC (Term `input (t:num) :bool`)
     THEN MATCH_ACCEPT_TAC th2
COMMENTS
To do similarly with several existentially quantified variables, use REPEAT_TCL CHOOSE_THEN ASSUME_TAC in place of CHOOSE_TAC
SEEALSO
CHOOSE_THEN, X_CHOOSE_TAC, REPEAT_TCL
HOL  Kananaskis-14