code: @Override
      public T visitUnaryExpression(UnaryExpression elm, C context) {
          if (elm instanceof CalculateAge) return visitCalculateAge((CalculateAge)elm, context);
          else return super.visitUnaryExpression(elm, context);
      }
docs: Visit a UnaryExpression. This method will be called for
  every node in the tree that is a UnaryExpression.
  
  @param elm     the ELM tree
  @param context the context passed to the visitor
  @return the visitor result