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.

The commodity rout is also putting pressure on stronger coal companies, which have weathered a difficult time for the sector in the last two years that forced smaller companies, such as Walter Energy Inc, into bankruptcy protection and more are expected to follow.

Answer: product_or_material_produced