Cond-expand and macros were designed for that job.  I'm going to be implementing

a library in the future that can use either bytevectors, u8vectors, or u1vectors through

exactly those means.  Bytevectors and u8vectors, which hopefully will be the

same thing on most Schemes (Guile is an exception), use 8 times as much

space, but they are more efficient to read and write one element at a time

than u1vectors are, at least on normal architectures.