docs: protected void validate( QueryContext context,
                               QueryCommand query,
                               Map<SelectorName, Table> usedSelectors ) {
          // // Resolve everything ...
          // Visitors.visitAll(query, new Validator(context, usedSelectors));
          // Resolve everything (except subqueries) ...
          Validator validator = new Validator(context, usedSelectors);
          query.accept(new WalkAllVisitor(validator) {
              @Override
              protected void enqueue( Visitable objectToBeVisited ) {
                  if (objectToBeVisited instanceof Subquery) return;
                  super.enqueue(objectToBeVisited);
              }
          });
      }