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