[REBOL] Writing to a half-byte

[tim]

In Assembler class we called a *niblet*
the lower- or higher- order 4 bits in 
a byte. I'd like a function that would
would write a niblet in a char! type.
It would something like this:

>>niblet/low #"a" 2    ; write 0010 to low-order "niblet"
==#"b"                 ; changed low-order "niblet" = 2
>>niblet/high #"b" 7   ; write 0111 to high-order "niblet"        
== #"r"                ; r is ascii 72

This is for a high-school course that I'm designing as
an intro to programming using rebol. 

Thanks:
-Tim