changed additional references by line from String to List and

implemented it on Item

library/src/main/java/org/mustangproject/Item.java

@@ -465,6 +465,29 @@ public class Item implements IZUGFeRDExportableItem {
 		return referencedDocuments.toArray(new IReferencedDocument[0]);
 	}
 
+
+	/***
+	 * adds item level references along with their typecodes and issuerassignedIDs (contract ID, cost centre, ...) 
+	 * @param doc the ReferencedDocument to add
+	 * @return fluent setter
+	 */
+	public Item addAdditionalReference(ReferencedDocument doc) {
+		if (referencedDocuments == null) {
+			referencedDocuments = new ArrayList<>();
+		}
+		referencedDocuments.add(doc);
+		return this;
+	}
+
+	@Override
+	public IReferencedDocument[] getAdditionalReferences() {
+		if (referencedDocuments == null) {
+			return null;
+		}
+		return referencedDocuments.toArray(new IReferencedDocument[0]);
+	}
+	
+	
 	/***
 	 * specify a item level delivery period
 	 * (apart from the document level delivery period, and the document level

library/src/main/java/org/mustangproject/ZUGFeRD/IZUGFeRDExportableItem.java

@@ -91,10 +91,10 @@ public interface IZUGFeRDExportableItem extends IAbsoluteValueProvider{
 	}
 
 	/***
-	 * the ID of an additionally referenced document for this item
+	 * allows to specify multiple references (billing information)
-	 * @return the id as string
+	 * @return the referenced documents
 	 */
-	default String getAdditionalReferencedDocumentID() {
+	default IReferencedDocument[] getAdditionalReferences() {
 		return null;
 	}
 

library/src/main/java/org/mustangproject/ZUGFeRD/ZUGFeRD1PullProvider.java

@@ -387,8 +387,8 @@ public class ZUGFeRD1PullProvider extends ZUGFeRD2PullProvider {
 					+ "<ram:LineTotalAmount currencyID=\"" + trans.getCurrency() + "\">" + currencyFormat(lc.getItemTotalNetAmount())
 					+ "</ram:LineTotalAmount>"
 					+ "</ram:SpecifiedTradeSettlementMonetarySummation>";
-			if (currentItem.getAdditionalReferencedDocumentID() != null) {
+			if (currentItem.getAdditionalReferences() != null) {
-				xml += "<ram:AdditionalReferencedDocument><ram:ID>" + currentItem.getAdditionalReferencedDocumentID() + "</ram:ID><ram:TypeCode>130</ram:TypeCode></ram:AdditionalReferencedDocument>";
+				xml += "<ram:AdditionalReferencedDocument><ram:ID>" + currentItem.getAdditionalReferences()[0].getIssuerAssignedID() + "</ram:ID><ram:TypeCode>130</ram:TypeCode></ram:AdditionalReferencedDocument>";
 
 			}
 			xml += "</ram:SpecifiedSupplyChainTradeSettlement>"

library/src/main/java/org/mustangproject/ZUGFeRD/ZUGFeRD2PullProvider.java

@@ -503,9 +503,14 @@ public class ZUGFeRD2PullProvider implements IXMLProvider {
 					+ "<ram:LineTotalAmount>" + currencyFormat(lc.getItemTotalNetAmount())
 					+ "</ram:LineTotalAmount>" // currencyID=\"EUR\"
 					+ "</ram:SpecifiedTradeSettlementLineMonetarySummation>";
-				if (currentItem.getAdditionalReferencedDocumentID() != null) {
+				if (currentItem.getAdditionalReferences() != null) {
-					xml += "<ram:AdditionalReferencedDocument><ram:IssuerAssignedID>" + currentItem.getAdditionalReferencedDocumentID() + "</ram:IssuerAssignedID><ram:TypeCode>130</ram:TypeCode></ram:AdditionalReferencedDocument>";
+					for (final IReferencedDocument currentReference : currentItem.getAdditionalReferences()) {
-
+						xml += "<ram:AdditionalReferencedDocument>" +
+							"<ram:IssuerAssignedID>" + XMLTools.encodeXML(currentReference.getIssuerAssignedID()) + "</ram:IssuerAssignedID>" +
+							"<ram:TypeCode>130</ram:TypeCode>" +
+							"<ram:ReferenceTypeCode>" + XMLTools.encodeXML(currentReference.getReferenceTypeCode()) + "</ram:ReferenceTypeCode>" +
+							"</ram:AdditionalReferencedDocument>";
+					}
 				}
 				xml += "</ram:SpecifiedLineTradeSettlement>"
 					+ "</ram:IncludedSupplyChainTradeLineItem>";