### Why don't log rules work for symbolic values?

Log rules are working for actual values, but fail for symbolic ones. What am I doing wrong?

```
var('a b')
c = bool(log(a*b) == log(a) + log(b));print(c)
c = bool(log(5*3) == log(5) + log(3));print(c)
c = log(5*3).n(); print(c)
c = log(5).n() + log(3).n(); print(c)
```

False
False
2.70805020110221
2.70805020110221