Fix a bug in the fresh var names generation : I was suffixing a var name by an incremented integer.

E.g., when creating a fresh var using the name of "L1", I was creating a var names "_L11", and then
"_L12", and so on. But if a var is named "L11", "_L111" is created. If if "L1" is used to generate
11 frash vart names, the name "_L111" clashes !!!

The fix contist in adding an underscore before : "L1" produces "_L1_1", "_L1_2", ..., and "_L1_11".

nb : the naming scheme is still wrong for other reasons... Next fix.