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メモメモ　Nextafter float

func Nextafter(x, y float64) (r float64)

Nextafter returns the next representable float64 value after x towards y. Special cases:

	Nextafter64(x, x)   = x
     Nextafter64(NaN, y) = NaN
     Nextafter64(x, NaN) = NaN

NaN（Not a Number、非数、ナン）