X,Y,Z : reserved: transient objet denoted by a capital letter
a,b : binder polarity
-c : reserved: future use (lambda_delta 3)
+c : reserved: future use (\lambda\delta 3)
d : relocation depth
e : relocation height
f :
p,q : global reference position
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)
+t,u : local reference position level (de Bruijn's) (RTM)
v,w :
x,y,z : reserved: transient objet denoted by a small letter