'Merge another QuotaSet into this one.
Existing quotas are not overridden.'

def __add__(self, other):

    if (not isinstance(other, QuotaSet)):
        msg = ('Can    only    add    QuotaSet    to    QuotaSet,    but    received    %s    instead' % type(other))
        raise ValueError(msg)
    for item in other:
        if (self.get(item.name).limit is None):
            self.items.append(item)
    return self
