Extract functions Occurs and Subst in their dedicated file.
Reintroduce tests for functions moved in their dedicated folder.
Introduce the first version of `two_in_a_row' function and an
interesting type check problem arises when we attemp to make
the functor for the first version (ie, the one in which `is_first_in'
helper function is defined independently) consume the comparer function.
Proposed a working version for the first implementation of `two_in_a_row'
but we're not completely happy for the result.
Add a first attempt that use high order modules, but doesn't type checks :(