In this paper we describe a share package XMOD
of functions for computing with finite, permutation crossed modules,
cat1-groups and their morphisms,
written using the GAP group theory programming language.
The category XMod
of crossed modules is equivalent to the category
Cat1 of cat1-groups and we include functions
emulating the functors between these categories.
The monoid of derivations of a crossed module X,
and the corresponding monoid of sections of a cat1-group C,
are constructed using the Whitehead multiplication.
The Whitehead group of invertible derivations,
together with the group of automorphisms of X
are used to construct the actor crossed module of X
which is the automorphism object in XMod.
We include a table of the 350 isomorphism classes
of cat1-structures on groups of order at most 30.
