7.7.2.2 INV Function
  The INV function inverts a matrix. BASIC can invert a ma-
  trix only if its subscripts are identical and it can be reduced to
  the identity matrix by elementary row operations. The input
  matrix multiplied by the output matrix (its inverse) always
  gives the identity matrix as a result.
  MAT INPUT first_array(3,3)
  MAT PRINT first_array;
  PRINT
  MAT inv_array = INV (first_array)
  MAT PRINT inv_array;
  PRINT
  MAT mult_array = first_array * inv_array
  MAT PRINT mult_array;
  PRINT
  D = DET
  PRINT D

  Output
  ? 4,0,0,0,0,2,0,8,0
    4 0 0
    0 0 2
    0 8 0
    .25 0 0
    0 0 .125
    0 .5 0
    1 0 0
    0 1 0
    0 0 1
    -64