A call to RESORT_EXISTS_CONV f t strips the outer
existentially-quantified variables of t, giving a list vs, such that
t is of the form ?vs. body.  The list vs is then passed to the
function argument f.  The result of the call f vs is expected to
be a new list of variables vs', and the result of the conversion is
the theorem
   |- (?vs. body) <=> (?vs'. body)