T,U,V,W: term
X,Y,Z : reserved: transient objet denoted by a capital letter
-a,b :
-c : local dropping kind parameter (true = restricted, false = general)
+a :
+b : local dropping kind parameter (true = restricted, false = general)
+c : rt-reduction count parameter
d : term degree
e : reserved: future use (\lambda\delta 3)
f,g : local reference transforming map