'When L{HistoricRecvLine} receives a BACKSPACE keystroke it deletes
the character immediately before the cursor.'

def test_backspace(self):

    kR = (lambda ch: self.p.keystrokeReceived(ch, None))
    for ch in iterbytes('xyz'):
        kR(ch)
    self.assertEqual(self.p.currentLineBuffer(), ('xyz', ''))
    kR(self.pt.BACKSPACE)
    self.assertEqual(self.p.currentLineBuffer(), ('xy', ''))
    kR(self.pt.LEFT_ARROW)
    kR(self.pt.BACKSPACE)
    self.assertEqual(self.p.currentLineBuffer(), ('', 'y'))
    kR(self.pt.BACKSPACE)
    self.assertEqual(self.p.currentLineBuffer(), ('', 'y'))
