xml-focus: extract XML subtrees and preserve structure

Extract subtrees from XML trees, making sure that the overall structure is preserved.

Examples

Extract a portion of TEI document, from the element pb@n='1' to the element pb@n='2':

doc = Nokogiri::XML(%q{
<TEI>
	<text>
		<body>
			<sp>
				<speaker>First</speaker>
				<l n='1'>a</l>
				<l n='2'>a</l>
				<l n='3'>a</l>

				<pb n='1'/>

				<l n='4'>a</l>
			</sp>

			<sp>
				<speaker>Second</speaker>
				<l n='5'>a</l>
				<l n='6'>a</l>
				<l n='7'>a</l>
				<l n='8'>a</l>

				<pb n='2'/>
			</sp>

			<sp>
				<speaker>Third</speaker>
				<l n='9'>a</l>
				<l n='10'>a</l>
				<l n='11'>a</l>

				<pb n='3'>
			</sp>
		</body>
	</text>
</TEI>
})

# Select all t
portion = XML::Focus(doc, 'name()="pb" and @n="1"', 'name()="pb" and @n="2"')

puts portion.to_xml # =>

# <TEI>
# 	<text>
# 		<body>
# 			<sp>
# 				<pb n='1'/>
#
# 				<l n='4'>a</l>
# 			</sp>
#
# 			<sp>
# 				<speaker>Second</speaker>
# 				<l n='5'>a</l>
# 				<l n='6'>a</l>
# 				<l n='7'>a</l>
# 				<l n='8'>a</l>
#
# 				<pb n='2'/>
# 			</sp>
# 		</body>
# 	</text>
# </TEI>