call site 20 for execnet.Channel.send
execnet/testing/test_gateway.py - line 368
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   def test_confusion_from_os_write_stderr(self):
       channel = self.gw.remote_exec("""
               import os
               os.write(2, 'test')
               channel.send(channel.receive() * 6)
               channel.send(channel.receive() * 6)
           """)
->     channel.send(3)
       res = channel.receive()
       assert res == 18
       channel.send(7)
       res = channel.receive()
       assert res == 42