This function returns the value pointed to by an Alien pointer or
the value of an Alien array element. If a pointer, an optional
single index can be specified to give the equivalent of C pointer
arithmetic; this index is scaled by the size of the type pointed to.
If an array, the number of indices must be the same as the number of
dimensions in the array type.
deref can be set with
setf to assign a new value.
This function extracts the value of slot slot-name from the an
union. If struct-or-union is a
pointer to a structure or union, then it is automatically
dereferenced. This can be set with
setf to assign a new
value. Note that slot-name is evaluated, and need not be a
compile-time constant (but only constant slot accesses are