The non-exact algorithms for the VRPTW have been very active-
far more active than that of exact algorithms. The field of
approximation algorithms and heuristics one sometimes classifies
an algorithm as sequential and parallel. In a sequential
algorithm one route at a time is constructed, while a parallel
algorithm may build more routes at the same time. This conflicts
with the notion of a sequential algorithm and a parallel
algorithm. We will therefore avoid the use of this classification
of heuristics.