code: def concat_list(listA, listB, delim=' '):
      
  
      # Lists must be of equal length.
      if len(listA) != len(listB):
          raise IndexError('Input lists are not parallel.')
  
      # Concatenate lists.
      listC = []
      for i in xrange(len(listA)):
          app = listA[i] + delim + listB[i]
          listC.append(app)
  
      return listC
docs: Concatenate list elements pair-wise with the delim character
      Returns the concatenated list
      Raises index error if lists are not parallel