Make behaviour of `DigraphRemoveEdge{s}` consistent when removing no edges

[Peter-Ing]

Added an additional method to preserve properties in the case of empty removals.

This addresses issue #260. (Note from Wilf: currently only partially.)

[wilfwilson]

Oh wait, what is the relationship between this PR and #338?

[Peter-Ing]

Oh wait, what is the relationship between this PR and #338?

this request should have been an update to #338 but as it hasn't updated it and comments have been left here I have closed #338.

[james-d-mitchell]

@Peter-Ing I agree with @wilfwilson. I think the intended changes would be the following:

if digraph is immutable:

• DigraphRemoveEdges(digraph, [ ]) returns digraph
• DigraphRemoveEdge(digraph, [ ]) gives the current error (the argument is not valid)
• DigraphRemoveEdges(digraph, [[src, ran]]) returns an immutable copy of digraph only when [src, ran] is valid. If [src, ran] is not valid, then the function could either give an error, or just return digraph (i.e. not a copy)
• DigraphRemoveEdge(digraph, [src, ran]) returns an immutable copy of digraph only when [src, ran] is valid. If [src, ran] is not valid, then the function could either give an error, or just return digraph (i.e. not a copy)

I'm not sure which is preferable: giving an error or just returning digraph and doing nothing. The latter would be more consistent with the current behaviour, but the former would probably be more helpful. Any thoughts?

[wilfwilson]

I tend to prefer the former kind of behaviour, I think it is what most people (or at least I) would expect. The latter kind of behaviour seems like it's done for optimisation, letting you reduce code (not having a separate existence check first, in some cases) and save time (by not twice checking whether an edge exists ). Unless users ask for that kind of behaviour, I don't think we should proactively offer it.

[codecov[bot]]

Codecov Report

Merging #340 (3b09b45) into master (514f5c3) will decrease coverage by 1.67%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #340      +/-   ##
==========================================
- Coverage   96.98%   95.31%   -1.68%     
==========================================
  Files          45       50       +5     
  Lines       12635    13038     +403     
==========================================
+ Hits        12254    12427     +173     
- Misses        381      611     +230     

Impacted Files	Coverage Δ
gap/oper.gi	98.37% <100.00%> (+<0.01%)	:arrow_up:
src/bitarray.c	93.33% <0.00%> (-1.91%)	:arrow_down:
gap/attr.gi	99.31% <0.00%> (-0.69%)	:arrow_down:
src/planar.c	93.75% <0.00%> (-0.37%)	:arrow_down:
src/perms.c	77.77% <0.00%> (-0.35%)	:arrow_down:
src/homos-graphs.c	98.66% <0.00%> (-0.27%)	:arrow_down:
src/homos.c	99.08% <0.00%> (-0.07%)	:arrow_down:
src/cliques.c	99.26% <0.00%> (-0.04%)	:arrow_down:
gap/attr.gd	100.00% <0.00%> (ø)
src/perms.h	100.00% <0.00%> (ø)
... and 11 more

[github-actions[bot]]

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