The right-associative unary operator cast as can be used to
derive a new offset value having a different type from an existing
value.
For example, this is how we would create a signed 12-bit offset in units bytes:
(poke) 1024#b as offset<int<12>,B> 128 as int<12>#B
The same rules governing conversion of integers apply for the magnitude part. Depending on the unit, there can be truncation, like in:
(poke) 9#b as offset<int,B> 1#B