def incomplete_tasks
      # TODO: refactor with reject
      incomplete = []
      categories.each { |c| incomplete << c if c.incomplete? }
      incomplete.sort_by { |c| [-c.percent_incomplete, -c.total_min_incomplete] }
    end
def incomplete_tasks
      # TODO: refactor with reject
      incomplete = []
      categories.each { |c| incomplete << c if c.incomplete? }
      incomplete.sort_by { |c| [-c.percent_incomplete, -c.total_min_incomplete] }
    end
Returns an array of incomplete tasks. This array is sorted first by
 percentage incomplete, then by total number of minutes incomplete.