fn differ_conditions(&self) -> impl Iterator<Item = &str> {
        self.open_parent()
            .map(|m| Box::new(m.conditions()) as Box<dyn Iterator<Item = &str>>)
            .unwrap_or_else(|| Box::new(empty()))
            .skip(self.common_conditions().count())
    }
Calculate the layers that parent context differs from current context
packages/stylist-core/src/ast/context.rs