then summing out the dimensions whose subscripts are not part of
      the output.
  Optionally, the output subscripts can be explicitly defined by
  adding an arrow ('->') at the end of the equation followed by
  the subscripts for the output. For instance, the following
  equation computes the transpose of a matrix multiplication:
  'ij,jk->ki'. The output subscripts must appear at least once for
  some input operand and at most once for the output.

  Ellipsis ('...') can be used in place of subscripts to broadcast
  the dimensions covered by the ellipsis. Each input operand may
  contain at most one ellipsis which will cover the dimensions not
  covered by subscripts, e.g. for an input operand with 5
  dimensions, the ellipsis in the equation *'ab...c'* cover the
  third and fourth dimensions. The ellipsis does not need to cover
  the same number of dimensions across the "operands" but the
