bug fix: wrong extension when reading files. Also added some printf in the Why core, I known that Andrei does not like it but I guess he won't notice. Who reads the commit messages anyway? Also added comments to env.mli, the end user will thank me a lot for that! What else? Ah yes, make bench does not work at all, a reference
to Graph is missing. I heard that Andrei played with the use or not use of ocamlgraph library. Seems to be wrong. But anyway, if Andrei does not feel like doing make bench before committing, why should I do it myself? That's all folks