GSYM : thm -> thm
STRUCTURE
Conv
SYNOPSIS
Reverses the first equation(s) encountered in a top-down search.
DESCRIPTION
The inference rule GSYM reverses the first equation(s) encountered in a top-down search of the conclusion of the argument theorem. An equation will be reversed iff it is not a proper subterm of another equation. If a theorem contains no equations, it will be returned unchanged. 
    A |- ..(s1 = s2)...(t1 = t2)..
   --------------------------------  GSYM
    A |- ..(s2 = s1)...(t2 = t1)..

FAILURE
Never fails, and never loops infinitely.
EXAMPLE
- arithmeticTheory.ADD;
> val it = |- (!n. 0 + n = n) /\ (!m n. (SUC m) + n = SUC(m + n)) : thm

- GSYM arithmeticTheory.ADD;
> val it = |- (!n. n = 0 + n) /\ (!m n. SUC(m + n) = (SUC m) + n) : thm

SEEALSO
NOT_EQ_SYM, REFL, SYM
HOL  Kananaskis-14