# roman numeral problem solution from:
# http://www.johndcook.com/blog/2012/01/14/roman-numeral-puzzle/comment-page-1/

def roman_numerals():
    import itertools
    values = list('IVXLCDM')
    index = dict((j,i) for i,j in enumerate(values))
    prefix = {0: (1,2), 2: (3,4), 4: (5,6)}
    for i in xrange(2, 7):
        for perm in itertools.permutations(values[:7], i):
            for j, v in enumerate(map(index.get, perm[:-1])):
                vn = index[perm[j+1]]
                if vn > v and vn not in prefix.get(v, ()):
                    break
            else:
                values.append(''.join(perm))
    return values