def render(digits):
    
    '''This function converts its input, a string of decimal digits, into a
    barcode, using the interleaved 2 of 5 format. The input string must not
    contain an odd number of digits. The output is an SVG string.

    Wikipedia [ITF Format]: http://en.wikipedia.org/wiki/Interleaved_2_of_5
    '''

    top = '<svg height="58" width="{0}" style="background:white">'
    bar = '<rect x="{0}" y="4" width="{1}" height="50" style="fill:black"/>'
    barcode = [top.format(len(digits) * 14 + 24)]

    byte_map = {
        '0': (2, 2, 4, 4, 2), '1': (4, 2, 2, 2, 4),
        '2': (2, 4, 2, 2, 4), '3': (4, 4, 2, 2, 2),
        '4': (2, 2, 4, 2, 4), '5': (4, 2, 4, 2, 2),
        '6': (2, 4, 4, 2, 2), '7': (2, 2, 2, 4, 4),
        '8': (4, 2, 2, 4, 2), '9': (2, 4, 2, 4, 2)
        }

    byte = lambda i: byte_map[digits[i]]

    def encode(bits, i=0):

        '''Encodes a string of decimal digits. The output's a binary string,
        but as a list of ints, all 2s and 4s, rather than zeros and ones, as
        the bars in the SVG barcode are rendered 2px wide for zeros, 4px for
        ones.'''

        if i == len(digits): return bits

        for odd, even in zip(byte(i), byte(i+1)): bits.extend([odd, even])

        return encode(bits, i+2)

    def svg(bits, i=0, x=4):

        '''Converts the list of ints returned by `encode` to a list of SVG
        elements that can be joined to form an SVG string.'''

        if i == len(bits): return barcode + ['</svg>']

        odd, even = bits[i:i+2]
        barcode.append(bar.format(x, odd))

        return svg(bits, i+2, x+odd+even)

    # The call to `encode` passes in the itf start code, and the itf end
    # code's added to the list that `encode` returns. The complete list
    # is passed to `svg`, which returns a list of SVG element strings
    # that're joined with newlines to create the actual SVG output.

    return '\n'.join(svg(encode([2, 2, 2, 2]) + [4, 2, 2, 0]))