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    seq_from_spec( \(@site1, @site2) )
        ->seq_concatmap( sub { seq_zip(map seq(split//), @_) } )
        ->seq_foreach(   sub { $counts{"@_"}++ } );

    sub upto  { 1 .. $_[0]     }
    sub supto { seq(upto(@_)); }
...
    #  3 => 1
    #  4 => 2
    #  5 => 3