text: # Define a procedure, union,
  # that takes as inputs two lists.
  # It should modify the first input
  # list to be the set union of the two
  # lists. You may assume the first list
  # is a set, that is, it contains no 
  # repeated elements.
  
  def union(ls1, ls2):
      for s in ls2:
          if s not in ls1:
              ls1.append(s)
  
  # To test, uncomment all lines 
  # below except those beginning with >>>.
  
  #a = [1,2,3]
  #b = [2,4,6]
  #union(a,b)
  #print a 
  #>>> [1,2,3,4,6]
  #print b
  #>>> [2,4,6]
  