in the original 48K-ROM and does all kinds of strange things. https://skoolkit.ca/disassemblies/rom/asm/13491.html warns "For a successful return to BASIC, HL' must on exit from the machine code contain the address of the 'end-calc' instruction at 10072." Is this all there is to know? Do other registers as HL' and SP also need to be considered?
"Truth would quickly cease to be stranger than fiction, once we got used to it." - H.L. Mencken