The structure and array expanser was buggy in presence of polymorphic

nodes (e.g., map<<+,2>>). While fixing  that, I put all the functions
that deals with polymorphism into a new (Eponymous) dedicated module.

The idea to be able to expand polymorphic node is basically the same,
as the one for printing polymorphic  nodes: we need to wait until the
type is instanciated (in GetEff).  That delay is implemented by using
a stack of nodes.