Added operations

Sources/Units/Units.swift

@@ -339,6 +339,37 @@ public struct UnitValue<ValueType: ConvertibleToDouble>:
 		!(lhs < rhs) && !(lhs == rhs)
 	}
 
+	/// The result will have the `unit` of the left hand side value.
+	public static func - <T: Numeric>(lhs: UnitValue<T>, rhs: UnitValue<T>)
+		-> UnitValue<Double>
+	{
+		(lhs.value - rhs.value)[lhs.unit]
+	}
+
+	/// The result will have the `unit` of the left hand side value.
+	public static func + <T: Numeric>(lhs: UnitValue<T>, rhs: UnitValue<T>)
+		-> UnitValue<Double>
+	{
+		(lhs.value + rhs.value)[lhs.unit]
+	}
+
+	/// The result will have the `unit` of the left hand side value.
+	public static func * <T: Numeric>(lhs: UnitValue<T>, rhs: UnitValue<T>)
+		-> UnitValue<Double>
+	{
+		(lhs.value * rhs.value)[lhs.unit]
+	}
+
+	/// The result will have the `unit` of the left hand side value.
+	public static func / <T: Numeric>(lhs: UnitValue<T>, rhs: UnitValue<T>)
+		-> UnitValue<Double>
+	{
+		guard rhs.value != 0 else {
+			fatalError("Cannot divide by zero.")
+		}
+		return (lhs.value * rhs.value)[lhs.unit]
+	}
+
 	// MARK: - Subscripts for UnitValue
 	/// Allows converting the UnitValue to another Unit using subscript syntax.
 	/// Example: `tenKilos[.pound]`