**Abstract:**

We study the structure of computations by term rewrite rules, allowing
non-determinism and overlapping. Computations form, up to permutations
of rewritings, a complete partial order, and semantics is defined as
the set of results of terminating (i.e. maximal) computations. A
call-by-need computation rule is then introduced and we prove, by means
of a continuous projection over the complete partial order of
call-by-need computations, that this rule is correct for sequential systems.