code: def render_inner(self, token):
          
          rendered = [self.render(child) for child in token.children]
          return ''.join(rendered)
docs: Recursively renders child tokens. Joins the rendered
          strings with no space in between.
  
          If newlines / spaces are needed between tokens, add them
          in their respective templates, or override this function
          in the renderer subclass, so that whitespace won't seem to
          appear magically for anyone reading your program.
  
          Arguments:
              token: a branch node who has children attribute.