Many many improvements:

Many many improvements:
1) First version after the introduction of explicit substitutions and
   the change in the DTD and in the stylesheets.
2) ElimIntros (that didn't work) have been replace by ElimIntrosSimpl
   that now does what the name tells you.
3) Buttons to move to the next and previous open goal introduced.
4) Serious bug in reduce and in simpl fixed: they didn't handle the environment
   properly.
5) replace is now an hygher order function, parametrized w.r.t. the equality
   to use. In some cases the right equality is physical equality. This closes
   some residual bugs.