xor logical operatorLibrary   "xor" 
 xor(a, b) 
  xor: Exclusive or, or exclusive disjunction is a logical operation that is true if and only if its arguments differ (one is true, the other is false).
  Parameters:
     a : first argument
     b : second argument
  Returns: returns xor (true only if a and b are true, but not both)
Example:
true xor true = false
true xor false = true
false xor true = true
false xor false = false
