Property references

As mentioned in the introduction, GML allows geographic features to have complex or aggregate non-geometric properties. A problem thus arises about how such properties should be referenced in the various places where property references are required (e.g. query and filter expressions). A WFS must use XPath expressions, as defined in this document, for referencing the properties and sub-properties of a feature encoded as XML elements or attributes. Feature attributes can be a string or a simple XLink. This means that the server needs to parse resoruce reference while parsing XPath expression. To adapt to this requirement, all WFS implementation should provide a method named valueOf() to acquire the specific implementation of XPath access functions.