def getch(self):
self.pos += <NUM_LIT:1><EOL>return self.string[self.pos - <NUM_LIT:1>:self.pos]<EOL>
Get a single character and advance the scan pointer.

    >>> s = Scanner("abc")
    >>> s.getch()
    'a'
    >>> s.getch()
    'b'
    >>> s.getch()
    'c'
    >>> s.pos
    3