If c is a conversion that maps a term ``t`` to a theorem |- t = t',
where t' is alpha-equivalent to t, or if c raises the
UNCHANGED exception when applied to ``t``, then CHANGED_CONV c
is a conversion that fails when applied to the term ``t``. If c
maps ``t`` to |- t = t', where t' is not alpha-equivalent to
t, then CHANGED_CONV c also maps ``t`` to |- t = t'. That is,
CHANGED_CONV c is the conversion that behaves exactly like c,
except that it fails whenever the conversion c would leave its input
term unchanged (up to alpha-equivalence).
When CHANGED_CONV c t fails, it raises an exception HOL_ERR ...,
not UNCHANGED, since some enclosing functions handle the
UNCHANGED exception as though c had succeeded by returning
the theorem |- t = t.