docs: private ParsedSelectStmt getLeftmostSelectStmt() {
          assert (!m_children.isEmpty());
          AbstractParsedStmt firstChild = m_children.get(0);
          if (firstChild instanceof ParsedSelectStmt) {
              return (ParsedSelectStmt) firstChild;
          } else {
              assert(firstChild instanceof ParsedUnionStmt);
              return ((ParsedUnionStmt)firstChild).getLeftmostSelectStmt();
          }
      }