Why do I get a C syntax error on compiling a rotate left instruction asm("RLF aux, w");?

Not all 8-bit devices have rotate left (RLF) instructions. In many 8-bit devices, rotate left through carry (RLCF) and rotate left no carry (RLNCF) instructions are present instead of RLF. Refer to the "Instruction Set Summary" chapter of the device datasheet.

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