Attempts to apply a conversion; applies identity conversion in case of failure.
DESCRIBE
TRY_CONV c "t" attempts to apply the conversion c to the term "t"; if
this fails, then the identity conversion applied instead. That is, if c is a
conversion that maps a term "t" to the theorem |- t = t', then the
conversion TRY_CONV c also maps "t" to |- t = t'. But if c fails when
applied to "t", then TRY_CONV c "t" returns |- t = t.