XML::Document#xpath inconsistent use of assumed namespaces

document_namespace_guessing.rb

#!/usr/bin/env ruby

require 'rubygems'
require 'nokogiri'

xml = %Q{
  <root xmlns:example="http://example.org">
    <example:element>
      <example:child>
    </example:element>
  </root>
}

doc = Nokogiri::XML(xml)


# 1
puts
assumed_namespace_on_root = doc.root.xpath("example:element")
# found it, no problem
puts "doc.root.xpath('example:element') => #{assumed_namespace_on_root}"

# 2
puts
begin
  unspecified_namespace_on_root = doc.root.xpath('bad:element')
rescue Nokogiri::XML::XPath::SyntaxError => e
  puts "doc.root.xpath('bad:element') raises exception, great: #{e}"
end

# 3
puts
begin
  unspecified_namespace_on_document = doc.xpath('bad:element')
rescue Nokogiri::XML::XPath::SyntaxError => e
  puts "doc.xpath('bad:element') still raises exception, great: #{e}"
end

# 4
puts
assumed_namespace_on_doc = doc.xpath("example:element")
puts %Q{Calling doc.xpath('example:element') should:
          * EITHER use the implicit namespaces on doc.root, and return the same thing as doc.root.xpath('example:element') as in #1  (This is my preference)
          * OR raise an exception for unknown namespace as in #2 and #3.

       Instead, it simply returns an empty set, that does not seem right, it's not raising unspecified namespace, but is ALSO not succesfully using the namespace, this is a recipe for confusion:
}
puts assumed_namespace_on_doc.inspect  #  => []