code: def finish(self):
          
          if self.state is not State.NORMAL:
              raise ReadError(f'EOF while in {self.state.name} state')
          if self.stack:
              top = self.stack[-1]
              top.raise_unclosed_error()
docs: Ensures the reader is in clean state (no unclosed S-expression
          is currently being parsed).  Should be called after the last
          ``feed_line``.