's += s2 is a shorthand for s.merge(s2).
Examples
>>> s = Struct(a=10,b=30)
>>> s2 = Struct(a=20,c=40)
>>> s += s2
>>> sorted(s.keys())
[\'a\', \'b\', \'c\']'

def __iadd__(self, other):

    self.merge(other)
    return self
