class REXML::XPathParser

def get_namespace( node, prefix )

2. If no mapping was supplied, use the context node to look up the namespace
1. Use the supplied namespace mapping first.

The rules are:
Returns a String namespace for a node, given a prefix 
def get_namespace( node, prefix )
  if @namespaces
    return @namespaces[prefix] || ''
  else
    return node.namespace( prefix ) if node.node_type == :element
    return ''
  end
end