def parse_epic_link(el):
    """Extract key of epic this issue belongs to (if given), else ''.

    Example XML:
    <customfields>
      <customfield id="customfield_10730" key="com.pyxis.greenhopper.jira:gh-epic-link">
        <customfieldname>Epic Link</customfieldname>
        <customfieldvalues>
          <customfieldvalue>IRS-4</customfieldvalue>
        </customfieldvalues>
      </customfield>
      ...
    </customfields>
    """
    if (el is None):
        return('')
    for customfield in el.findall('./customfield'):
        if (customfield.attrib['id'] == "customfield_10730"):
            return(customfield.find('./customfieldvalues/customfieldvalue').text)
    return('')