code: func (w *QWriter) FPrec(f float64, prec int) {
  	bb, ok := w.w.(*ByteBuffer)
  	if ok {
  		bb.B = strconv.AppendFloat(bb.B, f, 'f', prec, 64)
  	} else {
  		w.b = strconv.AppendFloat(w.b[:0], f, 'f', prec, 64)
  		w.Write(w.b)
  	}
  }
docs: // FPrec writes f to w using the given floating point precision.