Extract
 Usage
• Extract[expr, list] extracts the part of expr at the position specified by list.
• Extract[expr, { ,  , ... }] extracts a list of parts of expr.
• Extract[expr, ... , h] extracts parts of expr, wrapping each of them with head h before evaluation.
Notes
• Extract[expr, {i, j, ... }] is equivalent to Part[expr, i, j, ... ]. • The position specifications used by Extract have the same form as those returned by Position, and used in functions such as MapAt and ReplacePart. • You can use Extract[expr, ... , Hold] to extract parts without evaluation. • If expr is a SparseArray object, Extract[expr, ... ] extracts parts in the corresponding ordinary array. • New in Version 3.
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