Utilize the input text as a context reference, choose the right relationship between Walter Energy and coal from the options. Options: product/material produced, manufacturer, distributed by, industry, position held, original broadcaster, owned by, founded by, distribution format, headquarters location, stock exchange, currency, parent organization, chief executive officer, director/manager, owner of, operator, member of, employer, chairperson, platform, subsidiary, legal form, publisher, developer, brand, business division, location of formation, creator.

Other recent coal bankruptcies include Walter Energy Inc. and Patriot Coal Corp, which filed its second chapter 11 bankruptcy petition on May 12.

Answer: product_or_material_produced