code: def from_mapping(cls, mapping):
  		
  		out = cls()
  		for elem, count in mapping.items():
  			out._set_count(elem, count)
  		return out
docs: Create a bag from a dict of elem->count.
  
  		Each key in the dict is added if the value is > 0.
  
  		Raises:
  			ValueError: If any count is < 0.