Left some comments on Rietveld; mostly documentation-related (and pointed out some typos).

I'm also +1 on having to specify a True value rather than just defining the variable at all (i.e. setting __ltrace__ to False is the same as not defining it). If you change this, I'd probably force the value to be an actual bool, i.e. True or False (much like __debug__; except that it's read-only and a syntax error to assign to it).