code: def four_blocks(topleft, topright, bottomleft, bottomright):
      
      return vstack(hstack(topleft, topright),
                    hstack(bottomleft, bottomright))
docs: Convenience function that creates a block matrix with the specified
      blocks.
  
      Each argument must be a NumPy matrix. The two top matrices must have the
      same number of rows, as must the two bottom matrices. The two left matrices
      must have the same number of columns, as must the two right matrices.