type.go
func NewRat(a, b int64) *Rat {
	return &Rat{
		Rat: big.NewRat(a, b),
	}
}
func NewRat(a, b int64) *Rat {
	return &Rat{
		Rat: big.NewRat(a, b),
	}
}
// NewRat returns a new Rat.
// This is the similar to NewRat of math/big.