: zig ( p1 p2 -- p3 )
    [
        [ [ first ] [ second ] bi* + ]
        [ [ second ] [ first ] bi* - ] 2bi + 2 /
    ] [
        [ [ second ] [ first ] bi* - ]
        [ [ first ] [ second ] bi* + ] 2bi + 2 /
    ] 2bi 2array ;

: zag ( p1 p2 -- p3 )
    [
        [ [ first ] bi@ + ] [ [ second ] bi@ + ] 2bi - 2 /
    ] [
        [ [ first ] bi@ - ] [ [ second ] bi@ + ] 2bi + 2 /
    ] 2bi 2array ;