Get the set of consensus-generating fields for an operation.

def op_get_consensus_fields( op_name ):
    """
    Get the set of consensus-generating fields for an operation.
    """
    global SERIALIZE_FIELDS
    
    if op_name not in SERIALIZE_FIELDS.keys():
        raise Exception("No such operation '%s'" % op_name )

    fields = SERIALIZE_FIELDS[op_name][:]
    return fields