Getting a total from a user defined variable

[Joel via Digitalmars-d-learn]

```d
import std;

struct Person {
    string name;
    ulong age;
}
void main() {
    auto p=[Person("Joel", 43), Person("Timothy", 40)];
    writeln("Total: ", p.reduce!((a,b) => a.age+b.age)(0UL)); // 
how do I get the total of ages added together?
}
```