Comparable is the Mix-in for comparable classes. The
including class must provide the basic comparison operator
`<=>'. The other comparing operators are defined in this
module using operator `<=>'.
self > other
Returns true if self is greater than the other.
self >= other
Returns true if self is greater than or equals to the
other.
self < other
Returns true if self is less than the other.
self <= other
Returns true if self is less than or equals to the
other.
selfがotherより小さいか等しい時真を返す．
between?(min, max)
Returns true if self is in between min and max.