code: def snap_momentum_by_name(self, name, velocity, at=None):
          
          at = now_or(at)
          self.forget_past(at=at)
          return self.update_momentum_by_name(name, velocity=velocity, since=at)
docs: Changes the velocity of a momentum named `name`.
  
          :param name: the momentum name.
          :param velocity: a new velocity.
          :param at: the time to snap. (default: now)
  
          :returns: a momentum updated.
  
          :raises TypeError: `name` is ``None``.
          :raises KeyError: failed to find a momentum named `name`.