public SqlRexConvertlet get(SqlCall call) {
    SqlRexConvertlet convertlet;
    final SqlOperator op = call.getOperator();

    // Is there a convertlet for this operator
    // (e.g. SqlStdOperatorTable.plusOperator)?
    convertlet = (SqlRexConvertlet) map.get(op);
    if (convertlet != null) {
      return convertlet;
    }

    // Is there a convertlet for this class of operator
    // (e.g. SqlBinaryOperator)?
    Class<? extends Object> clazz = op.getClass();
    while (clazz != null) {
      convertlet = (SqlRexConvertlet) map.get(clazz);
      if (convertlet != null) {
        return convertlet;
      }
      clazz = clazz.getSuperclass();
    }

    // Is there a convertlet for this class of expression
    // (e.g. SqlCall)?
    clazz = call.getClass();
    while (clazz != null) {
      convertlet = (SqlRexConvertlet) map.get(clazz);
      if (convertlet != null) {
        return convertlet;
      }
      clazz = clazz.getSuperclass();
    }
    return null;
  }