code: def githubtunnel(user1, server1, user2, server2, port, verbose, stanford=False):
      
      if stanford:
          port_shift = 1
      else:
          port_shift = 0
  
      # command1 = 'ssh -nNf -L {}:quickpicmac3.slac.stanford.edu:22 {}@{}'.format(port, user, server)
      command1 = 'ssh -nNf -L {}:{}:22 {}@{}'.format(port-1-port_shift, server2, user1, server1)
      command2 = 'ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no -nNf -L {}:cardinal.stanford.edu:22 -p {} {}@localhost'.format(port-port_shift, port-port_shift-1, user2)
      command3 = 'ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no -nNf -L {}:github.com:22 -p {} {}@localhost'.format(port, port-1, user2)
      if verbose:
          print(command1)
          if stanford:
              print(command2)
          print(command3)
      try:
          call(shlex.split(command1))
          if stanford:
              call(shlex.split(command2))
          call(shlex.split(command3))
      except:
          print('Failure!')
          pass
docs: Opens a nested tunnel, first to *user1*@*server1*, then to *user2*@*server2*, for accessing on *port*.
  
      If *verbose* is true, prints various ssh commands.
  
      If *stanford* is true, shifts ports up by 1.
  
      Attempts to get *user1*, *user2* from environment variable ``USER_NAME`` if called from the command line.