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.

Walter Energy Inc., an Alabama-based coal miner, filed for bankruptcy Wednesday in what’s just the latest development in the collapse of U.S. metallurgical coal.

Answer: product_or_material_produced