code: public final UnresolvedReferenceExpression mergeOperand(UnresolvedReferenceExpression aggBuffer) {
  		String name = String.valueOf(Arrays.asList(aggBufferAttributes()).indexOf(aggBuffer));
  		validateOperandName(name);
  		return new UnresolvedReferenceExpression(name);
  	}
docs: Merge input of {@link #mergeExpressions()}, the input are AGG buffer generated by user definition.