maintaining (ugly and error-prone) hash tables.

That change revealed an untriggered bug in EvalClock.check_args: it was wrong
to add in subst the substitutions made of the parameters and the arguments (it
is enough to unify the clocks of the pars and of the args). For instance, consider
the node (in should_work/clock/clock.lus)

   node clock5(x :  bool; y:  bool when x; z: bool when y)

and the call

  z2 = clock5(a, b when a, c when e);

I was adding y/b in the subst, which was wrong.

Other minor changes:
 - move const_to_val_eff from Eff to UnifyClock.
 - GetEff.translate_val_exp now returns a substitution, in order to be able
   to unify clock vars and propagate the resulting substitution.

not expanded.

info is attached to all  val_exp. Moreover, this allow to perfrom all
the  type checking  in  EvalType.f, instead  of  doin part  of it  in
GetEff.translate_val_exp.

maintaining (ugly and error-prone) hash tables.

instanciated by a polymorphic operator.

extern constants.

as extern constants.

Also  fix  a  bug   in  the  struct/array  expanser:  when  expanding
structured constants  into val_exp,  I need to  add a entry  into the
type and clock val_exp tables.

 - expr such as "current x,y" should be written "current (x,y)"
 - abstract   struct  or   array   types  were   handled  as   extern
   types, which  prevent the struct  and array expanser to  work them
   out. In order to fix that, I have added an Abstract_type_eff variant
   to Eff.type_ which contains the concerte type.

2 source transformations), we add an entry in the EvalType and the EvalClock
tables. Otherwise, when we use EvalType.lookup or EvalClock.lookup, an error
migth be raised.

i.e., transform

      x,y = (0,1) -> (a,b);

into

      x = 0 -> a;
      y = 1 -> b;

where generated in the --expand-nodes mode..

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.

an expanser to will expand only struct, and i whant to name it like that).