code: def _code_line(self, line):
          
          assert self._containers
          container = self._containers[-1]
          # Handle extra spaces.
          text = line
          while text:
              if text.startswith('  '):
                  r = re.match(r'(^ +)', text)
                  n = len(r.group(1))
                  container.addElement(S(c=n))
                  text = text[n:]
              elif '  ' in text:
                  assert not text.startswith(' ')
                  i = text.index('  ')
                  container.addElement(Span(text=text[:i]))
                  text = text[i:]
              else:
                  container.addElement(Span(text=text))
                  text = ''
docs: Add a code line.