def category_ids(entries)
      category_ids = []
      entries.each do |day, tasks|
        category_ids += tasks.keys
      end
      category_ids.uniq
    end
def category_ids(entries)
      category_ids = []
      entries.each do |day, tasks|
        category_ids += tasks.keys
      end
      category_ids.uniq
    end
Finds all unique category ids in a log or a schedule
 @param entries [Hash] A parsed log or schedule
 @return [Array, String] Unique category ids