'Add clients from \'other\' _WindowCollection to self.
\'offset_to_current\' works as described for add()'

def join(self, other, offset_to_current=0):

    pos = max(0, (self.current_index + offset_to_current))
    if (pos < len(self.clients)):
        self.clients = ((self.clients[:pos:None] + other.clients) + self.clients[pos::None])
    else:
        self.clients.extend(other.clients)
