substitute with predicate won't compile

[Bastiaan Veelo]

Why doesn't this compile?

```d
import std;

void main()
{
     string s;
     //...
     s = s.substitute!((a, b) => sicmp(a, b) == 0, "&", "&",
                                                   "<", "<",
                                                   ">", ">",
                                                   """, `"`,
                                                   "'", 
"'").array;
}
```

-- Bastiaan.