Executing a ``catchpad`` instruction constitutes "entering" that pad.
The pad may then be "exited" in one of three ways:
+
1) explicitly via a ``catchret`` that consumes it. Executing such a ``catchret``
is undefined behavior if any descendant pads have been entered but not yet
exited.
Executing a ``cleanuppad`` instruction constitutes "entering" that pad.
The pad may then be "exited" in one of three ways:
+
1) explicitly via a ``cleanupret`` that consumes it. Executing such a ``cleanupret``
is undefined behavior if any descendant pads have been entered but not yet
exited.