PAIR_CONVPairRules.PAIR_CONV : (conv -> conv)
Applies a conversion to all the components of a pair structure.
For any conversion c, the function returned by
PAIR_CONV c is a conversion that applies c to
all the components of a pair. If the term t is not a pair,
them PAIR_CONV c t applies c to
t. If the term t is the pair
(t1,t2) then PAIR c t recursively applies
PAIR_CONV c to t1 and t2.
The conversion returned by PAIR_CONV c will fail for the
pair structure t if the conversion c would
fail for any of the components of t.