B(t)  uses { p ,p ,p ,p  }                                                  
              1  2  3  4                                                    
B'(t)  uses { v ,v ,v  },  where v  = 3(p -p ), v  = 3(p -p ), v  = 3(p -p )
               1  2  3            1      2  1    2      3  2    3      4  3