What is the relationship between Walter Energy and coal in the context of the input sentence. Choose an answer from: 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