code: public static Val exec(String rapids) {
      Session session = new Session();
      try {
        H2O.incrementActiveRapidsCounter();
        AstRoot ast = Rapids.parse(rapids);
        Val val = session.exec(ast, null);
        // Any returned Frame has it's REFCNT raised by +1, and the end(val) call
        // will account for that, copying Vecs as needed so that the returned
        // Frame is independent of the Session (which is disappearing).
        return session.end(val);
      } catch (Throwable ex) {
        throw session.endQuietly(ex);
      }
      finally {
        H2O.decrementActiveRapidsCounter();
      }
    }
docs: Execute a single rapids call in a short-lived session
  @param rapids expression to parse