code: def standardize_variables(sentence, dic=None):
      
      if dic is None: dic = {}
      if not isinstance(sentence, Expr):
          return sentence
      elif is_var_symbol(sentence.op):
          if sentence in dic:
              return dic[sentence]
          else:
              v = Expr('v_%d' % standardize_variables.counter.next())
              dic[sentence] = v
              return v
      else:
          return Expr(sentence.op,
                      *[standardize_variables(a, dic) for a in sentence.args])
docs: Replace all the variables in sentence with new variables.
      >>> e = expr('F(a, b, c) & G(c, A, 23)')
      >>> len(variables(standardize_variables(e)))
      3
      >>> variables(e).intersection(variables(standardize_variables(e)))
      set([])
      >>> is_variable(standardize_variables(expr('x')))
      True