It was because of my special system. The printed representation of unify variables
that point to a point to a point to a symbol got printed as a symbol. I really need to tweak
the output to hint if it is a unify variable. And if wingo managed to squeeze in tagging - so
much sweeter.
Now, the subtypes have been tested a little, the rest remains to be debugged.
Also there is a weak key map of functions to types of the object that I use to
record type signatures.