I,J : item
K,L : local environment
M,N : reserved: future use
-O,P,Q :
-R : generic predicate (relation)
+O :
+P : relocation environment
+Q : elimination predicate
+R : generic predicate/relation
S : RTM stack
T,U,V,W: term
X,Y,Z : reserved: transient objet denoted by a capital letter