ProjectionMap( map )
For each image i
under the (necessarily not parametrized) map map,
ProjectionMap( map )[i]
is the smallest preimage of i.
(We have CompositionMaps( map, ProjectionMap( map ) )
the identity map.)
gap> ProjectionMap( [1,1,1,2,2,2,3,4,5,5,5,6,6,6,7,7,7] ); [ 1, 4, 7, 8, 9, 12, 15 ]
GAP 3.4.4