How to use custom class in Angular service?

Question

Angular

There is a service:

@Injectable({ providedIn: "root" })
export class LayersSemantic {

}

And also a custom class:

export class A {
   do() {}
}

How to use class A in a service?

There are other options besides creating an instance:

@Injectable({ providedIn: "root" })
export class LayersSemantic {
   private a: A;
   constructor0 {
         this.a = new A();
   }
}