do_ leads to incosistency

Any unary lazy statement in `boost::phoenix` can be used like:
```C++
template
void call_lzay_func(const I& i, const F& f)
{
std::vector vec;
f[push_back(ref(vec), i)]();
}
int i;
f(ref(i), for_(ref(i) = 0, ref(i) < 5, ref(i)++));
```
The only exception is the `do_` statement.

When I have a function taking a unary `boost::phonenix` lazy statement (like you can see in the example), I have to make a falling distinctive to handle the `do_` specially.

I can't see any benefit in this. In which this simply change would solve the problem, without changing the semantic:
```C++
do_
[
cout << arg1 << ", "
]
.while_(arg1--),
```
to
```C++
do_(arg1--)
[
cout << arg1 << ", "
]
```
Isn't the current implementation incosistent in the manner of my example?