Expr value = new Value(7.0);
System.out.println(value.eval()); // affiche 7.0
Expr add = new Add(new Value(3.0), value);
System.out.println(add.eval());   // affiche 10.0
Expr expr = new Add(new Value(2.0), add);
System.out.println(expr.eval());  // affiche 12.0