code: def string(self):
          r
          if isinstance(self.expr, TexCmd) and len(self.expr.args) == 1:
              return self.expr.args[0].value
docs: r"""This is valid if and only if
  
          1. the expression is a :class:`.TexCmd` AND
          2. the command has only one argument.
  
          :rtype: Union[None,str]
  
          >>> from TexSoup import TexSoup
          >>> soup = TexSoup(r'''\textbf{Hello}''')
          >>> soup.textbf.string
          'Hello'
          >>> soup.textbf.string = 'Hello World'
          >>> soup.textbf.string
          'Hello World'
          >>> soup.textbf
          \textbf{Hello World}