Syntax fixes: not operator, until with numbers, list indeces with number...

Fixes #36, #29, and #22.

@marianoapp: Unfortunately I just discovered that we were both working on issue #22 at the same time, so we have different approaches to the same fix, and yours fix got merged about 1 hour ago while I was working on mine.

My fix to #22 contains the following things:
- Any arbitrary expression works, i.e. ```print plist#(x+3)^2.```  This allows an array of indeces into another array, even, like so: ```print plist#(myindeces#2).```
- floats convert to ints for the sake of indexing, i.e. ```print plist#2.0.``` is the same as ```print plist#2.```

I haven't looked at your fix yet to see what I did that may conflict.

Is there some way to record "someone is already working on this" so two people don't try to edit the same thing at the same time?  At any rate let me know if this will be a merging problem.