What is a "comma expression"?

[rempas]

This is the first time I'm finding something like that. I'm 
having the following code in C:

```
   if (0 == (i >> 7)) {
     if (len < 1) return -1;
     v = i;
     return *ret = v, 1;
   }
```

This is part of a function that returns an `int`. When I'm 
running this in C, it works. However in D, I'm getting the 
following error message:

```
Error: Using the result of a comma expression is not allowed
```

Can someone explain what comma expressions are and why they don't 
work in D (or if the only doesn't work as returned values from a 
function)?