code: def add_poly_index(self, i, j, k):
          
          self.idx_data.write(
              struct.pack(self.idx_fmt, i) +
              struct.pack(self.idx_fmt, j) +
              struct.pack(self.idx_fmt, k)
          )
docs: Add 3 ``SCALAR`` of ``uint`` to the ``idx_data`` buffer.