Notion of redundancy for APT transformations.
A call of an APT transformation is redundant if and only if
it is identical to a previous successful call of the same transformation
whose :show-only input is not t,
except that the two calls may differ in
their :print and :show-only inputs.
These options do not affect the generated events,
and thus they are ignored for the purpose of redundancy.
A call of an APT transformation whose :show-only input is t
does not generate any event.
No successive call may be redundant with such a call.