Mixin helper help

[John Chapman via Digitalmars-d-learn]

I'm obviously doing something wrong, but don't quite understand.
```d
mixin template helper() {
  mixin("writeln(12);");
}

struct Foo {
  void opDispatch(string name)() {
    import std.stdio;
    mixin helper!();
    //mixin("writeln(12);");
  }
}

void main() {
  Foo.init.opDispatch!"bar"();
}
```

The compiler emits these errors about the mixin ("writeln(12);"):
unexpected `(` in declarator
basic type expected, not `12`
found `12` when expecting `)`
no identifier for declarator `writeln(_error_)`
semicolon expected following function declaration
declaration expected, not `)`

Why does the commented code work but the mixin not? Thanks for 
any pointers.