REVISED version of workaround for call-by-visit

A student in the course has read the published work-around for call-by-visit carefully - and been puzzled by the name "caller" in the document. Five years after its original publication, I then discover that it should be "callee" instead!

I have revised the work-around for call-by-visit and it is now available in version 2, which has a better name for the variable!

-- Eric