Now notations are pretty printed in the same way as they are parsed.
Before "let x := e1 in e2" was notation for "(fun x => e2) e1",
resulting in overlapping notations for the same thing.

It is now slightly below implication. In order to do this, I had to change
the notation from P ={E1,E2}=> Q to P >{E1,E2}=> Q because the prefer ={n
is already used at level 70 for the distance of the metric.

We can use a named representation because we only substitute closed values. This
idea is borrowed from Pierce's Software Foundations.

The named representation has the following advantages:
* Programs are much better readable than those using De Bruijn indexes.
* Substitutions on closed terms (where all variables are explicit strings) can
  be performed by a mere simpl instead of Autosubst's asimpl. The performance
  of simpl seems better than asimpl.
* Syntactic sugar refolds better.