[bug] bad cases about \\sqrt and function

```
>>> parse('$\\sqrt{1/n}$'), parse('$\\frac{1}{\\sqrt{n}}$')
([sqrt(1/n), '\\sqrt{1/n}'], [1/sqrt(n), '\\frac{1}{\\sqrt{n}}'])

>>> verify(parse('$\\sqrt{1/n}$'), parse('$\\frac{1}{\\sqrt{n}}$'))
False

>>> parse('f(x) = 0'), parse('\\forall x>0, f(x) = 0')
([0, '0'], [0, '0'])

>>> verify(parse('f(x) = 0'), parse('\\forall x>0, f(x) = 0'))
True
```