find the shelf the user has added a book to

[Gunther]

There is already a possibility to add/remove a book to/from a users shelf using the shelves.add_to_shelf endpoint,
but as far as i know there is no possibility to see on which shelf an user has already added the book, short from getting all the user's shelves an the books on them

posibilities:
1)
endpoint: book.user_shelf
parameters: book_id
remark: requires OAuth and only searches the authenticated user's shelves

2)
endpoint: book.user_shelf
parameters: api_key, book_id, user_id
remarks:
- requires no OAuth
- allows you to check any users shelves if the book is on there, i dont think this is bad since you can already fetch a different user's shelves using shelves.list, and the books on those shelves using reviews.list (requires OAuth depending on the user's settings, might need to take this in consideration aswell)



possible response:

<?xml version="1.0" encoding="UTF-8"?>
<GoodreadsResponse>
	<Request>
		<authentication>true</authentication>
		<key>
			<![CDATA[xxxxxxxxxxxxxxxxxxx]]>
		</key>
		<method>
			<![CDATA[book_user_shelf]]>
		</method>
	</Request>
	<shelves start="1" end="3" total="3">
		<!-- left empty if none found -->
		<user_shelf>
			<id type="integer">6720933</id>
			<name>read</name>
			<book_count type="integer">0</book_count>
			<exclusive_flag type="boolean">true</exclusive_flag>
			<description nil="true" />
			<sort nil="true" />
			<order nil="true" />
			<per_page type="integer" nil="true" />
			<display_fields />
			<featured type="boolean">true</featured>
			<recommend_for type="boolean">true</recommend_for>
			<sticky type="boolean" nil="true" />
		</user_shelf>
		<user_shelf>
			<id type="integer">33789</id>
			<name>to-read</name>
			<book_count type="integer">0</book_count>
			<exclusive_flag type="boolean">true</exclusive_flag>
			<description nil="true" />
			<sort nil="true" />
			<order>a</order>
			<per_page type="integer" nil="true" />
			<display_fields />
			<featured type="boolean">false</featured>
			<recommend_for type="boolean">true</recommend_for>
			<sticky type="boolean" nil="true" />
		</user_shelf>
	</shelves>
</GoodreadsResponse>

Any comments?
Would it be possible to implement this?