FIRST_CONV
Conv.FIRST_CONV : conv list -> conv
Apply the first of the conversions in a given list that succeeds.
FIRST_CONV [c1,...,cn] t returns the result of applying to the term
t the first conversion ci that succeeds (or raises UNCHANGED) when
applied to t. The conversions are tried in the order in which they are
given in the list.
Failure
FIRST_CONV [c1,...,cn] t fails if all the conversions c1, ..., cn
fail when applied to the term t. FIRST_CONV cs t also fails if cs
is the empty list.