Design by Contract - out old struct member value

[Brother Bill]

Lets say we have the member function incrementBy(int increment)
```
struct Counter {
    int count;

    void incrementBy(int increment)
    in {
       assert(increment >= 0, "increment must be zero or greater");
    }
    out {
       // want to state  assert(count == old(count) + increment);
    }
    do {
       this.count += increment;
    }
}
```

old(count) is the value of count on entry to incrementBy()
old count is Eiffel syntax.

How is this done in D?