*** The function store-substring alters part of the contents<BR>
of a string.  You call it as follows:
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   (store-substring STRING IDX OBJ)
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This says to alter STRING, by storing OBJ starting at index IDX in
STRING.  OBJ may be either a character or a (smaller) string.
This function really does alter the contents of STRING.
Since it is impossible to change the length of an existing string,
it is an error if OBJ doesn't fit within STRING's actual length.
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