r a STOP opcode is delivered.  A triple is generated for
each opcode:

    opcode, arg, pos

opcode is an OpcodeInfo record, describing the current opcode.

If the opcode has an argument embedded in the pickle, arg is its decoded
value, as a Python object.  If the opcode doesn't have an argument, arg
is None.

If the pickle has a tell() method, pos was the value of pickle.tell()
before reading the current opcode.  If the pickle is a bytes object,
it's wrapped in a BytesIO object, and the latter's tell() result is
used.  Else (the pickle doesn't have a tell(), and it's not obvious how
to query its current position) pos is None.
)