K,L : local environment
M,N : reserved: future use
O :
-P,Q : reserved: future use
+P,Q : reserved: future use (lambda_delta 4)
R : generic predicate (relation)
S : RTM stack
T,U,V,W: term
X,Y,Z : reserved: transient objet denoted by a capital letter
+a,b : reserved: future use (lambda_delta 4)
+c : reserved: future use (lambda_delta 3)
d : relocation depth
e : relocation height
+f :
+g : sort degree parameter
h : sort hierarchy parameter
i,j : local reference position index (de Bruijn's)
k : sort index
+l : term degree
+m,n : reserved: future use
+o :
p,q : global reference position
+r,s :
t,u : local reference position level (de Bruijn's)
+v,w :
x,y,z : reserved: transient objet denoted by a small letter
NAMING CONVENTIONS FOR CONSTRUCTORS