Instead, assert that it is less than 0.

Otherwise, Coverity issues frequent complaints about dead code.

clang warns that a few implicit casts in trunnel truncate values from
64 to 32 bit. We do explicit casts to silence these warnings.

Back when spark was new, sticking patterns and productions in
docstrings was kinda reasonable.  But now that we have function
decorators in Python, I should use them: doing so will let me
actually put documentation on the methods of the lexer and parser.