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I am using string.find() in a situation where I don't know the index
(where the needle/pattern should match).  The index is arbitrary; +/-31 is
used in these examples.

This works as expected:

> = string.find('cat', 'cat', 31, true)

This does not:

> = string.find('cat', 'cat', -31, true)
1 3

I expect it to match the string at the index I passed to it, not
silently reposition the start at a valid index (1).  The function I think
needs fixing is str_find_aux() in lstrlib.c:

The first if-statement body should have this within it to correct the
behavior (replace the "init = 1"):

   lua_pushnil(L); /* cannot find anything */
   return 1;

If I am wrong and this was an intended behavior, then I would suggest
making it consistent -- if the 'init'/starting index is before or
after the string, make it a valid index in the string.  I'd rather
have it return
nil because the string doesn't "exist" at that point in memory,