isLegalTopLevelOpcode function
True when a specified opcode is itself a top-level record.
Attribute Stream is an embedded structure inside a Chunk or delta group, and Attachment Index remains reserved with an explicit writer prohibition. Keeping their registry names is useful to inspection output; it does not make either legal in the outer record sequence.
Implementation
bool isLegalTopLevelOpcode(int opcode) =>
isSpecifiedOpcode(opcode) &&
opcode != opAttributeStream &&
opcode != opAttachmentIndex;