pattern 3: cons
algebra-independent.
reduce(o, [3 [a b]], f) =
let (ra, f1) = reduce(o, a, f - 1)
let (rb, f2) = reduce(o, b, f1)
(cell(ra, rb), f2)
build a cell from two evaluated sub-expressions.
PARALLELISM: reduce(o,a) and reduce(o,b) are INDEPENDENT.
cost: 1. constraints: 1.