One thing I forgot to point out in my previous response is that the example you cite, in particular, is crippled by the small size of our vocabulary (to date). Had we the space to discuss concatmap, the following implementation would have been available:

    seq_from_spec( \(@site1, @site2) )
        ->seq_concatmap( sub { seq_zip(map seq(split//), @_) } )
        ->seq_foreach(   sub { $counts{"@_"}++ } );
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This implementation is much clearer (assuming you know concatmap).

Concatmap combines concat and map; it maps a sequence to a sequence of sequences, whose values are then concatenated to yield the output sequence. For example:

    sub upto  { 1 .. $_[0]     }
    sub supto { seq(upto(@_)); }

    seq(1..3) -> seq_map(\&upto) -> enumerate;
    #  0 => 1
    #  1 => 1 2
    #  2 => 1 2 3

    seq(1..3) -> seq_map(\&supto) -> seq_concat -> enumerate;
    #  0 => 1
    #  1 => 1
    #  2 => 2
    #  3 => 1
    #  4 => 2
    #  5 => 3

    seq(1..3) -> seq_concatmap(\&supto) -> enumerate;
    #  0 => 1
    #  1 => 1
    #  2 => 2
    #  3 => 1
    #  4 => 2
    #  5 => 3
[download]

Cheers,
Tom