code: def amalgamate(A, blocksize):
      
      if blocksize == 1:
          return A
      elif sp.mod(A.shape[0], blocksize) != 0:
          raise ValueError("Incompatible blocksize")
  
      A = A.tobsr(blocksize=(blocksize, blocksize))
      A.sort_indices()
      subI = (np.ones(A.indices.shape), A.indices, A.indptr)
      shape = (int(A.shape[0]/A.blocksize[0]),
               int(A.shape[1]/A.blocksize[1]))
      return csr_matrix(subI, shape=shape)
docs: Amalgamate matrix A.
  
      Parameters
      ----------
      A : csr_matrix
          Matrix to amalgamate
      blocksize : int
          blocksize to use while amalgamating
  
      Returns
      -------
      A_amal : csr_matrix
          Amalgamated  matrix A, first, convert A to BSR with square blocksize
          and then return a CSR matrix of ones using the resulting BSR indptr and
          indices
  
      Notes
      -----
      inverse operation of UnAmal for square matrices
  
      Examples
      --------
      >>> from numpy import array
      >>> from scipy.sparse import csr_matrix
      >>> from pyamg.util.utils import amalgamate
      >>> row = array([0,0,1])
      >>> col = array([0,2,1])
      >>> data = array([1,2,3])
      >>> A = csr_matrix( (data,(row,col)), shape=(4,4) )
      >>> A.todense()
      matrix([[1, 0, 2, 0],
              [0, 3, 0, 0],
              [0, 0, 0, 0],
              [0, 0, 0, 0]])
      >>> amalgamate(A,2).todense()
      matrix([[ 1.,  1.],
              [ 0.,  0.]])