p2+p2len            -3-2-1
p2                  +0+1+2
p1+p1len-6-5-4-3-2-1
p1      +0+1+2+3+4+5
         0 1 2 3 4 5 6 7 8 
        [a b c d e f 1 2 3] temp                           I     RHS
                    [1]>>>>>>[1]       temp   = *(p2+0)    0      *(p2+0)
              [d]>>>[d]               *(p2+0) = *(p1+3)    0      *(p1+(p1len-p2len))
        [a]>>>[a]                     *(p1+3) = *(p1+0)    0      *(p1+I)
        [1]<<<<<<<<<<<<<<<<<<[1]      *(p1+0) =  temp
                      [2]>>>>[2]       temp   = *(p2+1)    1      *(p2+I)
                [e]>>>[e]             *(p2+1) = *(p1+4)    1      *(p1+(p1len-p2len)+I)
          [b]>>>[b]                   *(p1+4) = *(p1+1)    1      *(p1+I)
          [2]<<<<<<<<<<<<<<<<[2]      *(p1+1) =  temp
                        [3]>>[3]       temp   = *(p2+2)    2      *(p2+I)
                  [f]>>>[f]           *(p2+2) = *(p1+5)    2      *(p1+(p1len-p2len)+I)
            [c]>>>[c]                 *(p1+5) = *(p1+2)    2      *(p1+I)
            [3]<<<<<<<<<<<<<<[3]      *(p1+2) =  temp    
        [1 2 3 a b c d e f]