FixedSubmodule( M )
returns the submodule of fixed points in the module M under the action of
the generators of M.ring.
gap> fix:= FixedSubmodule( nat );
Submodule( nat, [ [ Z(2)^0, Z(2)^0, Z(2)^0 ] ] )
gap> Dimension( fix );
1
GAP 3.4.4