Q.PAT_ABBREV_TAC : term quotation -> tactic
Introduces an abbreviation within the goal statement.
When applied to the goal (asl, w), the tactic Q.PAT_ABBREV_TAC q parses the quotation q in the context of the goal, producing an equality term v = p. The tactic then uses HolKernel.gen_find_term to search for a sub-term of w that p matches against. If such a sub-term t is found then all occurrences of t (in asl and w) will be replaced by v and an assumption Abbrev(v = t) is added to the goal.

The trace variable "PAT_ABBREV_TAC: match var/const" controls whether or not trivial matches (with constants or variables) are allowed. By default trivial matches are permitted but when the trace variable is false the tactic will ignore trivial matches, which could result in failure.

PAT_ABBREV_TAC fails if q does not successfully parse as an equality or if no matching sub-term is found in the goal, or if the only matching sub-terms would bind pattern variables to variables that are bound in the goal.
If the current goal is
   ?- (n + 10) * y <= 42315
then applying the tactic Q.PAT_ABBREV_TAC `X = a * b: num` results in the goal
   Abbrev (X = (n + 10) * y)
   X <= 42315
By default trivial matches are permitted. If the current goal is
   ?- y <= 42315
then Q.PAT_ABBREV_TAC `X = a: num` will give
   Abbrev (X = y)
   X <= 42315
However, if this is not desirable then setting
Feedback.set_trace "PAT_ABBREV_TAC: match var/const" 0
and applying Q.PAT_ABBREV_TAC `X = a: num` will give
   Abbrev (X = 42315)
   y <= X
If the current goal is
   ?- !x. x < 3
then applying Q.PAT_ABBREV_TAC `v = (a < b)` will result in a failure because the pattern’s variable a would have to bind the bound variable x in the goal.
HOL  Kananaskis-14