r(X,Y,Q,R) = { ( X, Y, X/Y, X mod Y ) | X >= 0 and Y > 0 }
( X,Y,Q,R := X,Y,0,R ) o ( X,Y,Q,R := X,Y,Q,X ) o again... now compose them [while R>=Y do Q := Q + 1 R := R - Y end]