Most Opcode.Label fields are just string-ified numbers for their index position. But sometimes, when they are the entry point for a function call (from a lock expression), the label is an identifier string. When this is the case, then the mere position of the opcode within the program is not enough to store the label. Therefore, for import/export to an ML file, in this case the numeric label needs to be stored. It is done by creating a dummy opcode that is just a no-op instruction intended to be removed when the program is actually loaded into memory and run. It exists purely to store, as an argument, the label of the next opcode to follow it.