substitute with predicate won't compile

[Bastiaan Veelo via Digitalmars-d-learn]

Why doesn't this compile?

```d
import std;
void main()
{
    string s;
    //...
    s = s.substitute!((a, b) => sicmp(a, b) == 0, "&", "&",
                                                  "&lt;", "<",
                                                  "&gt;", ">",
                                                  "&quot;", `"`,
                                                  "&apos;", 
"'").array;
}
```

-- Bastiaan.