As with REDUCE, the COMBINE Communication Primitive again
takes a shared input array 
and an associative operator
, and returns another 
shared array, consisting of
p copies of the value of 
, where 
uses
the associative operation 
. A simple implementation of this
primitive calls BCAST
, REDUCE
. Another
implementation follows from the stronger PREFIX primitive.
Instead of returning all p prefix-sums, only the value of 
is returned on each processor. Thus, the BDM model complexity analysis
is identical to Eq. (5).