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'e = r+w' means 'e' can contain the same descriptor twice, 
if it's both readable and writable, and I think this
matters.  Duplicate fd's don't cause a problem with the
select() system call itself, but the return value from
select.select() would include the file descriptor twice. 
(I've attached a test program that demonstrates this for
readable fds.)  

Therefore, I think the 'if is_r or is_w' logic is necessary.