     /  d          \ /  d          \     d   /                       \
     |-----        | |-----        |   ----- | -----                 |
     | \           | | \           |    \    |  \                    |
     |  )   x_i b_i| |  )   y_i b_i| =   )   |   )    x_i y_j h_ijk  | b_k
     | /           | | /           |    /    |  /                    |
     |-----        | |-----        |   ----- | -----                 |
     \i = 1        / \i = 1        /   k = 1 \  i,j                  /
    


         d                d                 d   
       -----            -----             -----  -----                 
        \                \                 \      \                    
 ==> (   )  x_i b_i )(    )  y_i b_i ) =    )  (   )  x_i y_j h_ijk ) b_k
        /                /                 /      /                    
       -----            -----             -----  -----                
       i = 1 i          i = 1             k = 1   i,j                
    
