Cat1EndomorphismSection( xi )
The endomorphisms gamma4, rho4 together determine a pair which may
be used to construct an endomorphism of {cal C}. When the
derivation is regular, the resulting morphism is an automorphism, and
this construction determines a homomorphism from the Whitehead group
to the automorphism group of C.
gap> psi4 := Cat1EndomorphismSection( xi4 );
Morphism of cat1-groups <[c3^2|Xc2 ==> s3]-->[c3^2|Xc2 ==> s3]>
GAP 3.4.4