m,n : reserved: future use
o :
p,q : global reference position
-r,s :
+r : reduction kind parameter (true = ordinary, false = extended)
+s : local dropping kind parameter (true = general, false = restricted)
t,u : local reference position level (de Bruijn's)
v,w :
x,y,z : reserved: transient objet denoted by a small letter
2: binary
A: application to vector
+F: boolean false
+T: boolean true
a: application
b: binder