* add param node in the TML with the name of the parameter
* add space node? active node?
* add alt boolean to all deleting methods
* implement a mechanism that allows the definition of new macros
* add open-group close-group attributes in the dictionary and handle them
* implement macro completion

BUGS
* $1\over{2}$ does not work

* compare pure_subst/pure_diff/subst_diff performances