code: public org.inferred.freebuilder.processor.property.Property.Builder mapBoxedType(
        UnaryOperator<TypeMirror> mapper) {
      return setBoxedType(getBoxedType().map(mapper));
    }
docs: If the value to be returned by {@link
  org.inferred.freebuilder.processor.property.Property#getBoxedType()} is present, replaces it by
  applying {@code mapper} to it and using the result.
  
  <p>If the result is null, clears the value.
  
  @return this {@code Builder} object
  @throws NullPointerException if {@code mapper} is null