Applies the first of two conversions that succeeds.
DESCRIBE
(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".
FAILURE
(c1 ORELSEC c2) "t" fails both c1 and c2 fail when applied to "t".