The REDUCE Communication Primitive takes a shared input array
and an associative operator 
, and on a single
processor, returns the value of 
, where 
uses the associative operation 
. We implement this primitive
by calling PREFIX with the array A and the operation 
.
Instead of using all p prefix-sums, only the value of 
is
returned. The BDM model complexity analysis is identical to
Eq. (5).