Equation:
  The "equation" string specifies the subscripts (letters in
  *[a-zA-Z]*) for each dimension of the input "operands" in the
  same order as the dimensions, separating subscripts for each
  operand by a comma (','), e.g. *'ij,jk'* specify subscripts for
  two 2D operands. The dimensions labeled with the same subscript
  must be broadcastable, that is, their size must either match or
  be *1*. The exception is if a subscript is repeated for the same
  input operand, in which case the dimensions labeled with this
  subscript for this operand must match in size and the operand
  will be replaced by its diagonal along these dimensions. The
  subscripts that appear exactly once in the "equation" will be
  part of the output, sorted in increasing alphabetical order. The
  output is computed by multiplying the input "operands" element-
  wise, with their dimensions aligned based on the subscripts, and
