7.7.1.2 Addition and Subtraction
  You can add the elements of two arrays. In the following
  statement, the two input lists, first_list% and second_list% ,
  must have identical dimensions. The elements of the new list,
  sum_list% , equal the sum of the corresponding elements in
  the input lists.
  MAT sum_list% = first_list% + second_list%

  You can also subtract the elements of two arrays. The fol-
  lowing program subtracts one array from another:
  DIM first_array(30,30)
  DIM second_array(30,30)
  DIM difference_array(30,30)
      .
      .
      .
  MAT difference_array = first_array - second_array

  Each element of difference_array is the arithmetic difference
  of the corresponding elements of the input arrays.