theorem
  |[ x1,x2,x3 ]| <X> |[ y1,y2,y3 ]|
  = |[ (x2*y3)-(x3*y2),(x3*y1)-(x1*y3),(x1*y2)-(x2*y1) ]|
proof
    set p1 = |[ x1,x2,x3 ]|;
    set p2 = |[ y1,y2,y3 ]|;
    thus thesis;
end;
