Use of 'xsd:ID' and 'xsd:IDREF'

Implementing more constrained XML types xsd:ID and xsd:IDREF instead of xsd:string in railML:tGenericID and railML:tGenericRef.

• That allows only one attribute of type xsd:ID in each element. railML schemas define railML:id attribute for this purpose, inherited by railML:tElementWithIDAndName.

• That requires unique railML:id attributes for whole XML file.

• That restricts railML:id attribute to pass the NCName rules: no Whitespaces, starting with a letter or "_", see also [http://www.w3.org/TR/1999/REC-xml-names-19990114/#NT-NCName]

• That requires railML:id with XML Attribute required or implied (no default possible).

• That requires non-zero-length attributes of type xsd:IDREF. railML schemas define attribute railML:refID, inherited by railML:tElementWithReference or railML:*RefId attributes for this reason.

• That requires xsd:IDREF attributes, that match an xsd:ID attribute value inside an XML file.

See also [http://www.w3.org/TR/2000/WD-xml-2e-20000814#NT-TokenizedType]

This type change results in stronger identifier and reference rules. XML validators are supposed to obey these rules and identify non-conforming XML files.