sub basic_tree_find {
    my ($tree_link, $target, $cmp) = @_;
    my $node;

    # $tree_link is the next pointer to be followed.
    # It will be undef if we reach the bottom of the tree.
    while ( $node = $$tree_link ) {
        local $^W = 0;      # no warnings, we expect undef values

        my $relation = ( defined $cmp
                    ? $cmp->( $target, $node->{val} )
                    : $target <=> $node->{val} );

        # If we found it, return the answer.
        return ($tree_link, $node) if $relation == 0;

        # Nope - prepare to descend further - decide which way we go.
        $tree_link = $relation > 0 ? \$node->{left} : \$node->{right};
    }

    # We fell off the bottom, so the element isn't there, but we
    # tell caller where to create a new element (if desired).
    return ($tree_link, undef);
}