Longer names are still accepted (as defined alias)

 NB : WC stands for WellFormed+Closed, here Closed in the sense "no free variables",
 i.e. FVars

 Also: some class overloading for fclosed, etc etc

  Moreover, it works both on term and formulas thanks to Class overloading

 These notations already exist in Coq 8.9, causing a nasty warning

 To be used later when interpreting lift_above formulas

 (and remove "auto with v62" in this file)

 This will be used to provide a "model" of our ZF theory

 See article :
 http://www.lix.polytechnique.fr/Labo/Benjamin.Werner/publis/tacs97.pdf