Routines
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7F56: Parser: step back to the previous frame
Decrements B (a frame counter), finds the previous frame (7F6F, 24 bytes higher) and swaps IX and IY, so that IY is the previous frame and IX the one we came from.
7F56 DEC B One frame fewer to go
7F57 CALL $7F6F IX = the previous frame (24 bytes higher)
7F5A JR $7F60 Swap IX and IY
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