sub map_pairs(&@) {
    my $cb = shift;

    my $caller = caller();

    my $ap = do { no strict 'refs'; \*{$caller.'::a'} };
    my $bp = do { no strict 'refs'; \*{$caller.'::b'} };

    local *$ap;
    local *$bp;

    my @res;
    while (@_) {
        *$ap = \shift;
        *$bp = \shift;
        push @res, &$cb;
    }
    return @res;
}