RCWA is a package for GAP 4. It provides implementations of algorithms and methods for computing in certain inﬁnite permutation groups acting on the set of integers. This package can be used to investigate the following types of groups and many more: • Finite groups, and certain divisible torsion groups which they embed into. • Free groups of ﬁnite rank. • Free products of ﬁnitely many ﬁnite groups. • Direct products of the above groups. • Wreath products of the above groups with ﬁnite groups and with (Z,+). • Subgroups of any such groups. With the help of this package, the author has found a countable simple group which is generated by involutions interchanging disjoint residue classes of Z and which all the above groups embed into – see [Koh10].