docs: public AbstractExpression getFirstArgument() {
          if (m_left != null) {
              assert(m_args == null);
              return m_left;
          }
          if (m_args != null && m_args.size() > 0) {
              assert(m_left == null && m_right == null);
              return m_args.get(0);
          }
          return null;
      }