sub operator:mat* is prec(\&operator:*) (@X, @Y) {
    return map {
        $^m1; # disambiguate ... right?
        reduce{[ @^a ^+ @^b ]}
            map { $X[$^m1][$^m2] ^* @{$Y[$^m2]} } 0 .. $#Y }
    } 0 .. $#X;
}