MapAt
 Usage
• MapAt[f, expr, n] applies f to the element at position n in expr. If n is negative, the position is counted from the end.
• MapAt[f, expr, {i, j, ... }] applies f to the part of expr at position {i, j, ... }.
• MapAt[f, expr, {{ ,  , ... }, { ,  , ... }, ... }] applies f to parts of expr at several positions.
Notes
• MapAt[f, expr, {i, j, ... }] or MapAt[f, expr, {{i, j, ... }}] applies f to the part expr[[i, j, ... ]]. • MapAt[f, expr, {{ ,  , ... }, { ,  , ... }, ... }] applies f to parts expr[[ ,  , ... ]], expr[[ ,  , ... ]], ... . • The list of positions used by MapAt is in the same form as is returned by the function Position. • MapAt applies f repeatedly to a particular part if that part is mentioned more than once in the list of positions. • New in Version 1.
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