[MLton] What are the semantics for Word_addCheck?

[Ben Chambers]

Also, what other primitives are we likely to come across in a simple program?
Basically, we've finished a CPS transform, and we're now trying to interpret
them in order to ensure that the transform was correct (before we start running
things like Delta CFA and Gamma CFA on them).  Most of the interpreter is
written except for the Prim Ops, and we're trying to figure out what those do
and how they work.  Obviously, if we don't need to do all of them, that would be
favorable.  If not, an explanation of those primitives we need to do would be
cool.

Thanks,
Ben Chambers