ORELSEC
op Conv.ORELSEC : (conv -> conv -> conv)
Applies the first of two conversions that succeeds.
(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``. (This refers to failure other than by raising
UNCHANGED).