Prevent event duplication

A quick fix to trim away extra events if they somehow wind up cloned
(EG, item.identify() and material.identify() both lining up)
An existent example: on player shoots bow: fires twice without this
commit.
Clean element.is.than error handling

Two fixes:
- 'less' is now permitted in addition to 'LESS' (case-insensitive)
- a fully bad operator (EG '.is[potato]') will respond with a simple
error message rather than a full stacktrace.

Also, player.target, return Element.NULL rather than a real null
pointer.