code: public AnnotatedTypeBuilder<X> addToField(Field field, Annotation annotation) {
         if (fields.get(field) == null) {
             fields.put(field, new AnnotationBuilder());
         }
         fields.get(field).add(annotation);
         return this;
     }
docs: Add an annotation to the specified field. If the field is not already
  present, it will be added.
  
  @param field      the field to add the annotation to
  @param annotation the annotation to add
  @throws IllegalArgumentException if the annotation is null