Given a JavaScript file, find the sourceMappingURL line

def discover(source):
    "Given a JavaScript file, find the sourceMappingURL line"
    source = source.splitlines()
    # Source maps are only going to exist at either the top or bottom of the document.
    # Technically, there isn't anything indicating *where* it should exist, so we
    # are generous and assume it's somewhere either in the first or last 5 lines.
    # If it's somewhere else in the document, you're probably doing it wrong.
    if len(source) > 10:
        possibilities = source[:5] + source[-5:]
    else:
        possibilities = source

    for line in set(possibilities):
        pragma = line[:21]
        if pragma == '//# sourceMappingURL=' or pragma == '//@ sourceMappingURL=':
            # We want everything AFTER the pragma, which is 21 chars long
            return line[21:].rstrip()
    # XXX: Return None or raise an exception?
    return None