Guidelines:
	• If the first operand is not an object of the class in all usages:   (e.g., + ) – overload it as a friend non-member • As a non-member, if the operands are not modified by the operator (and are objects of a class) – the arguments should be const references • If the first operand is always an object of the class: (+=) – overload it as a member • As a member, if the operator does not modify the current object (i.e., data members are not modified) – overload it as a  const member