On pure context-free language and left szilard languages

A corrugated sheet has a bend formed by one or more straight lines of impressions extending transversely across the corrugation valleys upwardly into the valleys and adjacent portions of the side walls between the valleys and the corrugation crests, and at the ends of the impressions the side walls have indents extending outwardly away from the impressions. The number of indents and impressions depends on the extent of the bend.