This attribute is relevant only when output is sent to a user-written ContentHandler,
that is, a SAXResult. It causes extra information to be maintained and made available to
the ContentHandler for diagnostic purposes: specifically, the Locator that is
passed to the ContentHandler via the setDocumentLocator method may be cast to a
ContentHandlerProxyLocator, which exposes the method getContextItemStack(). This
returns a java.util.Stack. The top item on the stack is the current context item, and below this
are previous context items. Each item is represented by the interface net.sf.saxon.om.Item. If
the item is a node, and if the node is one derived by parsing a source document with the line-numbering
option enabled, then it is possible to obtain the URI and line number of this node in the original XML source.
For this to work, the code must be compiled with tracing enabled. This can be achieved by setting the option
config.setCompileWithTracing(true) on the Configuration object, or equivalently by
setting the property FeatureKeys.COMPILE_WITH_TRACING on the JAXP TransformerFactory.
Note that this compile-time option imposes a substantial run-time overhead, even if tracing is not switched on
at run-time by providing a TraceListener.
