Stop the 'combination operations' from changing their given list of digraphs #468
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By 'combination operations', I mean
DigraphEdgeUnion
,DigraphJoin
,DigraphDisjointUnion
,DigraphDirectProduct
, andDigraphCartesianProduct
. See #467.Given a list of digraphs, these operations would try to change this list. Therefore, they would either wrongly mutate this list, if it was mutable, else they would give an error when this failed.
Note: this concerns mutability of lists, not digraphs.
To fix this, I made
DIGRAPHS_CombinationOperProcessArgs
copy its argument, and return that copy when it is finished.Resolves issue #467.