<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_resource(mt_file, ref, cache):
 """Add a resources entry, downloading the intuiting the file, replacing entries with the same reference"""


    if isinstance(mt_file, MetapackDoc):
        doc = mt_file
    else:
        doc = MetapackDoc(mt_file)

    if not 'Resources' in doc:
        doc.new_section('Resources')

    doc['Resources'].args = [e for e in set(doc['Resources'].args + ['Name', 'StartLine', 'HeaderLines', 'Encoding']) if
                             e]

    seen_names = set()

    u = parse_app_url(ref)

    # The web and file URLs don't list the same.

    if u.proto == 'file':
        entries = u.list()
    else:
        entries = [ssu for su in u.list() for ssu in su.list()]

    errors = []

    for e in entries:
        if not add_single_resource(doc, e, cache=cache, seen_names=seen_names):
            errors.append(e)

    if errors:
        prt()
        warn("Found, but failed to add these urls:")
        for e in errors:
            print('    ', e)

    write_doc(doc, mt_file)