@property<EOL><INDENT>def md5checksum(self):<DEDENT>
raise NotImplementedError("<STR_LIT>")<EOL>
Order-independent MD5 checksum which identifies this `ReferenceSet`.

To compute this checksum, make a list of `Reference.md5checksum` for all
`Reference`s in this set. Then sort that list, and take the MD5 hash of
all the strings concatenated together. Express the hash as a lower-case
hexadecimal string.