TAG is evalled to get the tag to use; it must not be nil.
Then the BODY is executed.
-Within BODY, (throw TAG) with same tag exits BODY and exits this `catch'.
+Within BODY, a call to `throw' with the same TAG exits BODY and this `catch'.
If no throw happens, `catch' returns the value of the last BODY form.
If a throw happens, it specifies the value to return from `catch'.
usage: (catch TAG BODY...) */)