call site 20 for execnet.Channel.receive
execnet/testing/test_gateway.py - line 225
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   def test_channel_callback_after_receive(self):
       l = []
       channel = self.gw.remote_exec(source='''
               channel.send(42)
               channel.send(13)
               channel.send(channel.gateway.newchannel())
               ''') 
->     x = channel.receive()
       assert x == 42
       channel.setcallback(callback=l.append)
       py.test.raises(IOError, channel.receive)
       channel.waitclose(TESTTIMEOUT) 
       assert len(l) == 2
       assert l[0] == 13
       assert isinstance(l[1], channel.__class__)