<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def from_etree(tree):
 """Constructs an executable form a given ElementTree structure. :param tree: :type tree: xml.etree.ElementTree.ElementTree :rtype: Executable """

        exe = Executable(tree)

        exe.category = tree.findtext('category')
        exe.version = tree.findtext('version')
        exe.title = tree.findtext('title') or exe.name
        exe.description = tree.findtext('description')
        exe.license = tree.findtext('license') or "unknown"
        exe.contributor = tree.findtext('contributor')

        for ps in tree.iterfind("parameters"):
            assert isinstance(ps, ET.Element)
            paras = ParameterGroup(
                ps.findtext("label"),
                ps.findtext("description"),
                ps.attrib.get('advanced', "false") == "true",
                filter(lambda x: x is not None,
                       map(Parameter.from_xml_node, list(ps))))

            exe.parameter_groups.append(paras)

        return exe