IRef refers to any memory location in any parts of the Mu memory,
including both heap, stack and global. The common part about them is
that Mu knows the types (at least the location of reference fields) of
all of them. We use the term "conceptually boxed" and "conceptually
unboxed" to describe the difference between the destination of "iref"
and "uptr": irefs always refer into "conceptually boxed" things (boxed
in the sense that Mu knows their field types (at least reference
fields)). Heap objects are obviously boxed. We can consider each stack
as a huge box, and the entire global space as a huge box. "iref" can
only be obtained from the allocation operations in Mu. But uptrs may
point outside the Mu memory, which can be literally anywhere. They can
even be created by C functions, such as "malloc".