input: Write a function to add two matrices. The function should first check if the matrices are compatible for addition. If the matrices have different dimensions, the function should return 'Matrices not compatible'. Otherwise, it should sum the corresponding elements of the matrices and return the resulting matrix.
  
  assert add_matrix([[1, 2], [3, 4]], [[5, 6], [7, 8]])==[[6, 8], [10, 12]]
  assert add_matrix([[1, 2, 3], [4, 5, 6]], [[7, 8, 9], [10, 11, 12]])==[[8, 10, 12], [14, 16, 18]]
  assert add_matrix([[1, 2], [3, 4]], [[5, 6, 7], [8, 9, 10]])=='Matrices not compatible'
  assert add_matrix([[1], [2]], [[3], [4]])==[[4], [6]]
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))]