Function depth_declarations 

pub fn depth_declarations(depth: Depth) -> String

The fill and edge declarations for a depth, as a rule body.

Empty for Depth::Flat, which has neither and inherits what it sits on. Callers lean on the emptiness to skip the rule rather than emit a class that sets nothing: a class that sets no properties is a class that means “I thought about this”, which is what comments are for.