Swift 4 で「プラマイ」範囲を作る ref: http://qiita.com/lovee/items/4159b5c64b371ecb0b8f

Operator.swift

infix operator ± : RangeFormationPrecedence

extension Numeric where Self: Comparable {

    public static func ± (lhs: Self, rhs: Self) -> ClosedRange<Self> {
        let added = lhs + rhs
        let subtracted = lhs - rhs
        let lowerBound = min(added, subtracted)
        let upperBound = max(added, subtracted)
        return lowerBound ... upperBound
    }

}