def to_filter_proc(filter)
        case ff = filter
        when NilClass then ->(f){ true }
        when Proc     then ff
        when Regexp   then ->(f){ ff =~ f.to_s }
        else
          ->(f){ ff === f }
        end
      end
def to_filter_proc(filter)
        case ff = filter
        when NilClass then ->(f){ true }
        when Proc     then ff
        when Regexp   then ->(f){ ff =~ f.to_s }
        else
          ->(f){ ff === f }
        end
      end
Returns a proc that implements file_filter strategy according to the
 type of filter installed