new_contents: # http://codingbat.com/prob/p143951
  
  def lone_sum(a, b, c):
    sum = 0
    
    if a != b and a != c:
      sum += a
    
    if b != a and b != c:
      sum += b
      
    if c != a and c != b:
      sum += c
      
    return sum
  
subject: Create lone_sum.py