class Fugit::Duration

def _to_s(key)
  KEYS.inject([ StringIO.new, '+' ]) { |(s, sign), (k, a)|
    v = @h[k]
    next [ s, sign ] unless v
    sign1 = v < 0 ? '-' : '+'
    s << (sign1 != sign ? sign1 : '') << v.abs.to_s << a[key]
    [ s, sign1 ]
  }[0].string
end; protected :_to_s