code: public final UnresolvedReferenceExpression operand(int i) {
  		String name = String.valueOf(i);
  		if (getAggBufferNames().contains(name)) {
  			throw new IllegalStateException(
  				String.format("Agg buffer name(%s) should not same to operands.", name));
  		}
  		return new UnresolvedReferenceExpression(name);
  	}
docs: Arg of accumulate and retract, the input value (usually obtained from a new arrived data).