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GAP 4.17 splits PreImagesRepresentative into a checked operation and PreImagesRepresentativeNC, and the library calls the latter. Semigroup homomorphisms by images only had a method for the checked operation, so PreImagesRepresentativeNC fell through to the library method for s.p. general mappings, which signals "no default method". Install the unchecked computation for PreImagesRepresentativeNC as well, keeping the checked method and its error message. On older GAP both names are the same operation, so the extra method is only installed when they differ. The inverse function of AsSemigroupIsomorphismByFunction used PreImages, which returns a list of preimages rather than an element, in every GAP version. Use PreImagesRepresentativeNC instead; the argument lies in the range of an isomorphism. The automorphism group computations for Rees (0-)matrix semigroups call PreImagesRepresentative on elements known to be in the image. Call PreImagesRepresentativeNC there, skipping the range and image membership tests that the checked operation performs in GAP 4.17. Assisted-by: Claude Code (Opus 5)
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GAP 4.17 splits PreImagesRepresentative into a checked operation and PreImagesRepresentativeNC, and the library calls the latter. Semigroup homomorphisms by images only had a method for the checked operation, so PreImagesRepresentativeNC fell through to the library method for s.p. general mappings, which signals "no default method". Install the unchecked computation for PreImagesRepresentativeNC as well, keeping the checked method and its error message. On older GAP both names are the same operation, so the extra method is only installed when they differ.
The inverse function of AsSemigroupIsomorphismByFunction used PreImages, which returns a list of preimages rather than an element, in every GAP version. Use PreImagesRepresentativeNC instead; the argument lies in the range of an isomorphism.
The automorphism group computations for Rees (0-)matrix semigroups call PreImagesRepresentative on elements known to be in the image. Call PreImagesRepresentativeNC there, skipping the range and image membership tests that the checked operation performs in GAP 4.17.
Assisted-by: Claude Code (Opus 5)