This is documentation for Mathematica 6, which was
based on an earlier version of the Wolfram Language.
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Parts of Expressions
Mathematica's unified symbolic architecture allows immediate generalization of part-oriented list operations to arbitrary expressions—supporting operations both on individual parts, and on collections of parts at specified levels in expression trees.
Part (..[[..]]) get any part of an expression (reset with =)
Length  ▪ First  ▪ Last  ▪ Most  ▪ Rest  ▪ Take  ▪ Drop  ▪ Extract
ReplacePart replace numbered parts of an expression
Append  ▪ Prepend  ▪ Insert  ▪ Delete  ▪ DeleteCases
    
Position find the positions of any pattern in an expression
Cases find cases of any pattern in an expression
    
MapAt apply a function at particular positions in an expression
Map (/@) — apply a function at specified levels in an expression
MapAll (//@) — apply a function to all parts of an expression
    
Heads option for specifying whether to include heads
    
Sow, Reap collect parts "sown" inside of an expression
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