'Parse an existing markdown changeset section and return the VersionLog instance.'

@classmethod
def from_md_lines(cls, lines):

    instance = cls()
    (instance.pre_header, version_header, tail) = isplit(u'##    ', lines)
    if version_header:
        instance.version_header = version_header
    (instance.post_header, section, tail) = isplit(u'###    ', tail)
    while section:
        (instance.sections[section], section, tail) = isplit(u'###    ', tail)
    return instance
