[ new ] Hereditary Subst. for TT

Implemented in Haskell. We have to use a trick to deal with the
fact that we cannot represent the idea of "removing [v] from the
context [gamma]" used in commit a22172cb3cf66d3adf00d5396fbfe75d2c72fdcb
But the idea is quite natural and everything works.

/!\ Code not tested