Fix ViewString method for inverse monoids without generators as an inverse semigroup/monoid

[james-d-mitchell]

Description of changes (for the release notes)

Previously for inverse monoids with GeneratorsOfSemigroup but not with GeneratorsOfInverseSemigroup the wrong method for ViewString was selected, i.e. the method for ViewString of an inverse monoid (with no generators) had higher priority than the intended method.

Before this commit:

gap> S := Monoid(Transformation([1, 2, 3, 4, 5, 6, 7, 7, 7]),
>                Transformation([4, 6, 3, 6, 6, 6, 7, 7, 7]),
>                Transformation([4, 5, 6, 1, 6, 6, 7, 7, 7]),
>                Transformation([6, 6, 3, 1, 6, 6, 7, 7, 7]),
>                Transformation([4, 6, 6, 1, 2, 6, 7, 7, 7]));;
gap> IsInverseSemigroup(S);
true
gap> S;
<inverse monoid>

after:

gap> S := Monoid(Transformation([1, 2, 3, 4, 5, 6, 7, 7, 7]), 
>                Transformation([4, 6, 3, 6, 6, 6, 7, 7, 7]),
>                Transformation([4, 5, 6, 1, 6, 6, 7, 7, 7]), 
>                Transformation([6, 6, 3, 1, 6, 6, 7, 7, 7]), 
>                Transformation([4, 6, 6, 1, 2, 6, 7, 7, 7]));;
gap> IsInverseSemigroup(S);
true
gap> S;
<inverse transformation monoid of size 18, degree 9 with 5 generators>

[codecov-io]

Current coverage is 49.06% (diff: 100%)

Merging #880 into master will increase coverage by <.01%

@@             master       #880   diff @@
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  Files           422        422          
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  Methods        3447       3447          
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  Branches          0          0          
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+ Hits         112221     112227     +6   
+ Misses       116490     116484     -6   
  Partials          0          0          

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[olexandr-konovalov]

@james-d-mitchell thanks, looks good to me.

[olexandr-konovalov]

@james-d-mitchell now we have a diff in the test with all packages loaded:

########> Diff in /mnt/disk2/hudson-slave/workspace/GAP-dev/GAPCOPTS/64build/G\
APGMP/gmp/GAPTARGET/standard/label/64bit/GAP-git-snapshot/tst/testinstall/semi\
grp.tst:456
# Input is:
S;
# Expected output:
<inverse transformation monoid of size 18, degree 9 with 5 generators>
# But found:
<inverse transformation monoid of degree 9 with 5 generators>
########

[james-d-mitchell]

This is (probably) caused by the different methods for IsInverseSemigroup in the Semigroups package and in GAP. This can be easily fixed by adding the line gap> Size(S);; to the test file before the S;.

Should I amend the PR accordingly?

[olexandr-konovalov]

@james-d-mitchell thanks - since the PR has been merged already, I can adjust the test myself.

[james-d-mitchell]

@alex-konovalov ok great!

[olexandr-konovalov]

Updated test example in 80a0966