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sub cross {
    my $f = shift or return;                  # Nothing.
...
    my @r = cross(@_);                        # Recurse.
    map {my $l = $_; map {[$l, @$_]} @r} @$f; # Distribute.
}

sub cross {
    my $f = shift or return [];               # Nothing.    
    my @r = cross(@_);                        # Recurse.
    map {my $l = $_; map {[$l, @$_]} @r} @$f; # Distribute.
}