unmap operator has the form:
It gets any value and produces the same value, making it not mapped in case it is a mapped value.
This is useful when we want to read a data structure from the IO space (say, an array of integers) and then use it for storage without changing the underlying IO space. We would do something like:
(poke) var a = unmap (int @ 10#B) (poke) a = 100