code: def submatrix(matrix,i1,i2,j1,j2):
      
  
      new = []
      for i in range(i1,i2+1):
          new.append(matrix[i][j1:j2+1])
      return _n.array(new)
docs: returns the submatrix defined by the index bounds i1-i2 and j1-j2
  
      Endpoints included!