\DOC ABBREV_TAC

\TYPE {ABBREV_TAC : term -> (string * thm) list * term -> goalstate}

\SYNOPSIS
Tactic to introduce an abbreviation.

\DESCRIBE
The tactic {ABBREV_TAC `x = t`} abbreviates any instances of the term {t} in
the goal (assumptions or conclusion) with {x}, and adds a new assumption {t =
x}. (Reversed so that rules like {ASM_REWRITE_TAC} will not immediately expand
it again.) The LHS may be of the form {f x} in which case abstraction will
happen first.

\FAILURE
Fails unless the left-hand side is a variable or a variable applied to a list
of variable arguments.

\EXAMPLE
{
  # g `(12345 + 12345) + f(12345 + 12345) = f(12345 + 12345)`;;
  Warning: Free variables in goal: f
  val it : goalstack = 1 subgoal (1 total)

  `(12345 + 12345) + f (12345 + 12345) = f (12345 + 12345)`

  # e(ABBREV_TAC `n = 12345 + 12345`);;
  val it : goalstack = 1 subgoal (1 total)

   0 [`12345 + 12345 = n`]

  `n + f n = f n`
}

\USES
Convenient for abbreviating large and unwieldy expressions as a sort of `local
definition'.

\SEEALSO
EXPAND_TAC.

\ENDDOC