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Add a deque and a bit of logic, and one would get a fifo
implementation of the cache.

from collections import deque
_toss = deque()

def _compile(fmt):
    # Internal: compile struct pattern
    if len(_cache) >= _MAXCACHE:
        del _cache[_toss.popleft()]
    s = Struct(fmt)
    _cache[fmt] = s
    _toss.append(fmt)
    return s

Race conditions from multiple threads could result in
#threads-1 more entries in the cache than _MAXCACHE, but
this could be trimmed in later calls by replacing 'if' with
'while'.