docs: def Add(self, other):
      """Add other to self pointwise.
  
      Requires that both self and other are of the same length, and contain
      identical timestamps. Typically this means that Normalize has been called
      on both with identical time parameters.
  
      Args:
        other: The sequence to add to self.
  
      Raises:
        RuntimeError: other does not contain the same timestamps as self.
      """
      if len(self.data) != len(other.data):
        raise RuntimeError("Can only add series of identical lengths.")
      for i in range(len(self.data)):
        if self.data[i][1] != other.data[i][1]:
          raise RuntimeError("Timestamp mismatch.")
        if self.data[i][0] is None and other.data[i][0] is None:
          continue
        self.data[i][0] = (self.data[i][0] or 0) + (other.data[i][0] or 0)