Addition and subtraction operators for Swift Optionals

Optional+Operators.swift

func +=<T>(lhs: inout T?, rhs: T) where T: SignedNumeric {
    switch lhs {
    case .none:
        break
    case .some(let value):
        lhs = .some(value + rhs)
    }
}

func -=<T>(lhs: inout T?, rhs: T) where T: SignedNumeric {
    lhs += -rhs
}

var hours: Int? = 5
hours -= 2
print(hours) //Prints Optional(3)