not that easy I'm still struggling to see the best API for a set of functionals operating on hashes.

for instance my function invert should be renamed because you can do %h2= invert invert %h such that you get the original back. so maybe hash_part or hpart might be better.

To have a real "mathematical" invert one has to consider a special structure relation which is effectively a HoA (simple hashes are "functions" which can only be inverted if they are bijective i.e. have unique values)

  DB<131> sub invert {
            my %h=@_;
            my %h2;
            while ( my ($k,$v) = each %h ) { 
              push @{ $h2{$_} } , $k for @$v;
            } 
            return %h2;
          }

  DB<134> %t = (1 => ["e", "c", "a"], 2 => ["b", "d", "f"], 3=>[a] )
 => (1, ["e", "c", "a"], 2, ["b", "d", "f"], 3, ["a"])

  DB<135> invert %t
 => ("e", [1], "c", [1], "a", [1, 3], "b", [2], "d", [2], "f", [2])

  DB<136> invert invert %t
 => (1, ["e", "c", "a"], 3, ["a"], 2, ["b", "d", "f"])
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Now taking the last use cases we saw, I'm not sure how this special application of invert would help.

AND I had to change the interface, to be chainable.

Originally I was operating on a hash_reference and not a list.

DWIM isn't easy here, prototypes don't allow mixing '%hashes' and LISTs, such that the former is taken as reference.

(One might pass an explicit reference '\%hash' and check if the LIST has just one element. Well not very elegant ...)

Hope you understand my struggles...