Undefined function in CBOR library

In CBOR library there are a couple of unidentified functions such as:

unpack_float16

def decode_float16(decoder):
    payload = decoder.read(2)
    return unpack_float16(payload)

and

undefined

special_decoders = {
    20: lambda self: False,
    21: lambda self: True,
    22: lambda self: None,
    23: lambda self: undefined,
    24: decode_simple_value,
    25: decode_float16,
    26: decode_float32,
    27: decode_float64,
    31: lambda self: break_marker,
}

Start by locating the CBOR library code containing decode_float16 and special_decoders, then trace where unpack_float16 and undefined are expected to come from. Reproduce the problem and inspect existing tests or decoding entry points; done means the referenced functions and value are resolved without breaking CBOR decoding.