input: Write a function to add two matrices. If the matrices are not of the same size, return 'Matrices not compatible'. Otherwise, return the resultant matrix obtained by adding corresponding elements of the input matrices.
  
  assert add_matrix([[1, 2], [3, 4]], [[5, 6], [7, 8]]) == [[6, 8], [10, 12]]
  assert add_matrix([[1, 2, 3], [4, 5, 6]], [[1, 1, 1], [1, 1, 1]]) == [[2, 3, 4], [5, 6, 7]]
  assert add_matrix([[1], [2]], [[1], [2]]) == [[2], [4]]
  assert add_matrix([[1, 2]], [[3, 4]]) == [[4, 6]]
  assert add_matrix([[1], [2]], [[3]]) == 'Matrices not compatible'
output: def add_matrix(mat1, mat2):
      if len(mat1) != len(mat2) or any(len(row1) != len(row2) for row1, row2 in zip(mat1, mat2)):
          return 'Matrices not compatible'
      return [[mat1[i][j] + mat2[i][j] for j in range(len(mat1[0]))] for i in range(len(mat1))]