### Similar limits give very different behavior

The following code produces the expected right answer:

```
sage: var('x,h')
sage: f(x) = sin(x^2)
sage: lim((f(x+h)-f(x))/h, h=0)
```

The answer, of course, is 2*x*cos(x^2). However, the following code fails to complete:

```
sage: lim((f(pi+h)-f(pi))/h, h=0)
```

Sage just thinks and thinks and does not come up with an answer. This happens on two different versions I have access to, 5.2 and 5.0. What is happening?