I,J : item
K,L : local environment
M,N : reserved: future use
-O :
-P,Q : reserved: future use (lambda_delta 4)
+O,P,Q :
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)
+a,b : binder polarity
c : reserved: future use (lambda_delta 3)
d : relocation depth
e : relocation height