SWAP_EXISTS_CONV : conv
STRUCTURE
SYNOPSIS
Interchanges the order of two existentially quantified variables.
DESCRIPTION
When applied to a term argument of the form ?x y. P, the conversion SWAP_EXISTS_CONV returns the theorem:
   |- (?x y. P) = (?y x. P)

FAILURE
SWAP_EXISTS_CONV fails if applied to a term that is not of the form ?x y. P.
HOL  Kananaskis-13