If the conversion c1 returns |- t = t' when applied to a term "t", and
c2 returns |- t' = t'' when applied to "t'", then the composite
conversion (c1 THENC c2) "t" returns |- t = t''. That is,
(c1 THENC c2) "t" has the effect of transforming the term "t"
first with the conversion c1 and then with the conversion c2.
FAILURE
(c1 THENC c2) "t" fails if either the conversion c1 fails when applied to
"t", or if c1 "t" succeeds and returns |- t = t' but c2 fails when
applied to "t'". (c1 THENC c2) "t" may also fail if either of c1 or c2
is not, in fact, a conversion (i.e. a function that maps a term t to a
theorem |- t = t').