Doubly Nested Anonymous Classes

The code below is from Effective Java, item 18. Can anyone explain why the
book and the comment refer to "doubly nested anonymous classes"? When I
compile the program the names I see have only one dollar sign in them. I
can see they are anonymous claases but why "doubly nested"? Thanks.

// Typical use of a public static member class - Page 94
public class Calculator {
   public static abstract class Operation {
      private final String name;

      Operation(String name) { this.name = name; }

      public String toString() { return this.name; }

      // Perform arithmetic op represented by this constant
      abstract double eval(double x, double y);

      // Doubly nested anonymous classes
      public static final Operation PLUS = new Operation("+") {
         double eval(double x, double y) { return x + y; }
      };
      public static final Operation MINUS = new Operation("-") {
         double eval(double x, double y) { return x - y; }
      };
      public static final Operation TIMES = new Operation("*") {
         double eval(double x, double y) { return x * y; }
      };
      public static final Operation DIVIDE = new Operation("/") {
         double eval(double x, double y) { return x / y; }
      };
   }

   // Return the results of the specified calculation
   public double calculate(double x, Operation op, double y) {
      return op.eval(x, y);
   }
}