equality on filters incorrect

[ChrisJefferson]

(IsPermGroup and IsTransitive) = (IsPermGroup and IsTransitive) currently returns false.

How such things should be compared is non-trivial -- in particular as we add implications the answer could change. Therefore I suggest we just add an overload which Errors.

[ChrisJefferson]

By the answer could change, I mean consider we have:

InstallTrueMethod( IsSmallList, IsList and IsCanonicalBasisFullMatrixModule );

After this is installed, should IsSmallList and IsSmallList and IsList compare as equal or not? I don't think there is a good answer.

[fingolfin]

It's a matter of how one wants to define "equality". One possible definition is: two filters are equal if their underlying flag sets are equal:

gap> FLAGS_FILTER(IsPermGroup and IsTransitive) = FLAGS_FILTER(IsPermGroup and IsTransitive);
true

BTW, IsSmallList always implies IsList. But this implication does not change IsList, and hence:

gap> FLAGS_FILTER(IsSmallList) = FLAGS_FILTER(IsSmallList and IsList);
false

And hence there is stability with this definition of equality. Of course one may not care about that and might prefer "logical" equality which tracks all implications, then one could use WITH_IMPS_FLAGS, e.g.

gap> WITH_IMPS_FLAGS(FLAGS_FILTER(IsSmallList)) = WITH_IMPS_FLAGS(FLAGS_FILTER(IsSmallList and IsList));
true