def merge_dictionaries(D1,D2):
    """merge_dictionaries(D1,D2)
  
       Merge two disjoint dictionaries
       The first argument D1 is modified to hold the result of the merger.
       If dictionaries are not disjoint an exception is raised
    """
    import types

    for k in D2.keys():
        val = D2[k]
        if not D1.has_key(k):
            D1[k] = val        
        else:
            print 'Dictionaries not disjoint'
            raise Exception
  
    return D1