First partial syncronization between the HTML and the MathML presentation:

First partial syncronization between the HTML and the MathML presentation:
many csymbols added to MathML presentation.

*** NOTE! ***
The code seems to work perfectly, but I am unsure it is well implemented.
It is better to have a thorough look at it as soon as possible.
*************

The syncronization is still incomplete. The following csymbols are
missing (all of them from the Huet contrib):

subst
lift lift_with_base
beta_red1
beta_red
par_beta_red1
par_beta_red
forgetful
isomorphic
interp