The symbolic context of a region R is an absolute map 
describing
each live variable's value on entry to R in terms of loop invariants
and loop indices of enclosing loops.
maps
all variables to Unknown.
), identity (
): as for relative maps.
is applied
to an absolute map 
to derive a new absolute map:
.
: map actuals to
formals everywhere in 
.
: Eliminate from the map all bindings
of variables with no upwards-exposed reads in Proc.
: Only identical maps are equivalent.