Applies the first of two conversions that succeeds.
DESCRIPTION
(c1 ORELSEC c2) ``t`` returns the result of applying the conversion
c1 to the term ``t`` if this succeeds. Otherwise
(c1 ORELSEC c2) ``t`` returns the result of applying the conversion
c2 to the term ``t``. If either conversion raises the UNCHANGED
exception when applied, this is passed on to ORELSEC’s caller.
FAILURE
(c1 ORELSEC c2) ``t`` fails if both c1 and c2 fail when applied
to ``t``.