\DOC SYM

\TYPE {SYM : thm -> thm}

\SYNOPSIS
Swaps left-hand and right-hand sides of an equation.

\KEYWORDS
rule, symmetry, equality.

\DESCRIBE
When applied to a theorem {A |- t1 = t2}, the inference rule {SYM} returns
{A |- t2 = t1}.
{
    A |- t1 = t2
   --------------  SYM
    A |- t2 = t1
}


\FAILURE
Fails unless the theorem is equational.

\SEEALSO
Conv.GSYM, Drule.NOT_EQ_SYM, Thm.REFL.
\ENDDOC