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FlattenAt

FlattenAt[list, n]
flattens out a sublist that appears as the nth element of list. If n is negative, the position is counted from the end.
FlattenAt[expr, {i, j, ...}]
flattens out the part of expr at position {i, j, ...}.
FlattenAt[expr, {{i1, j1, ...}, {i2, j2, ...}, ...}]
flattens out parts of expr at several positions.
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