Good old BBC Basic and its handling of variables

During the weekend I spent some time making James Reynolds !Spectrum work on recent machine and centralized the code for effect selection and initialization which was duplicated in several places. So ended with something looking like this:

DIM n% 256,...

DEF PROCAss
...
.buf EQUD n%
...
END PROC

DEFPROCselect_effect(n%)
PROCAss
...
ENDPROC

Having meaningful variable names would make the issue less likely too.

Because of course it ended using the value passed to PROCselect_effect() for the EQUD

Of course it did. That was what you asked it to do.