title: incrementing negatively in a for loop does not provide the correct answer
body: <p>Given the following code which counts positively for set A and negatively for set B . Set A works as expected but set B gives <code>0</code> as answer instead of <code>-2</code>. Why is that ?</p>
  
  <pre><code>arr = [1,3,2,12]
  A = set([1,3,5])
  B = set([1,3,5])
  
  def func(arr,A,B):
      countA = 0
      countB = 0
      for i in arr:
          if i in A:
              countA += 1
          elif i in B:
              countB -= 1
  
      return countA
  </code></pre>
  
  <p>Returns 2. But....</p>
  
  <pre><code>arr = [1,3,2,12]
  A = set([1,3,5])
  B = set([1,3,5])
  
  def func(arr,A,B):
      countA = 0
      countB = 0
      for i in arr:
          if i in A:
              countA += 1
          elif i in B:
              countB -= 1
  
      return countB
  </code></pre>
  
  <p>Returns 0</p>