code: public final UnresolvedReferenceExpression[] operands() {
  		int operandCount = operandCount();
  		Preconditions.checkState(operandCount >= 0, "inputCount must be greater than or equal to 0.");
  		UnresolvedReferenceExpression[] ret = new UnresolvedReferenceExpression[operandCount];
  		for (int i = 0; i < operandCount; i++) {
  			String name = String.valueOf(i);
  			validateOperandName(name);
  			ret[i] = new UnresolvedReferenceExpression(name);
  		}
  		return ret;
  	}
docs: Args of accumulate and retract, the input value (usually obtained from a new arrived data).