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Diversions are a way of temporarily saving output. The output of
m4 can at any time be diverted to a temporary file, and be
reinserted into the output stream, undiverted, again at a later
time.
Numbered diversions are counted from 0 upwards, diversion number 0
being the normal output stream. The number of simultaneous diversions
is limited mainly by the memory used to describe them, because GNU
m4 tries to keep diversions in memory. However, there is a
limit to the overall memory usable by all diversions taken altogether
(512K, currently). When this maximum is about to be exceeded,
a temporary file is opened to receive the contents of the biggest
diversion still in memory, freeing this memory for other diversions.
So, it is theoretically possible that the number of diversions be
limited by the number of available file descriptors.