Breakpoints on lines with generator expressions

Say I have

``` py
if all(i == j for i, j in zip(range(10), range(10))):
print("yes")
```

and put a breakpoint on the first line. If I then `c`ontinue to the breakpoint and press `n`, it steps inside the generator expression. A similar thing happens with

``` py
if (lambda: True)():
print("yes")
```

but not for something like

``` py
if test():
print("yes")
```

The first one is an issue because of https://github.com/inducer/pudb/issues/141, but in general, this behavior seems counterintuitive. I would expect `n` to step over the line and `s` to step into it, which is what happens when you reach the line via normal means.

Reproduce the generator-expression and lambda examples in a debugging session, comparing `n` with `s` after continuing to the breakpoint. Trace the debugger's stepping decision for those expressions; done means `n` steps over the current line while `s` enters it, without changing the existing behavior for a normal function call.

Written by the indexing model from the issue text.