p0 = (A, B, C, D);          Call match(p0)
    x1 <= A; p1 = (B, C, D);  p.pop
    x1 <= A; y1 <= B;         valid pair, call match(C,D)
      x2 <= C; p2 = (D);      p.pop
      x2 <= C; y2 <= D;       invalid pair - return FAIL
    x1 <= A; y1 <= C;         valid pair, call match(B,D)
      x2 <= B; p2 = (D);      p.pop
      x2 <= B; y2 <= D;       valid pair, call match()
        p3 = ()               p3.empty - return SUCCESS
      r <= ((B, D));          Push results - return SUCCESS
    r <= ((B, D), (A, C))     Push results - return SUCCESS
  SUCCESS
  Results are B&D, A&C