This is documentation for Mathematica 7, which was
based on an earlier version of the Wolfram Language.
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Expression Structure
A foundational idea in Mathematica is that all expressions—whatever they may represent—ultimately have a uniform tree-like structure.
FullForm the full functional form of an expression, without shortened syntax
TreeForm a representation of the tree structure of small expressions
    
Head the head of an expression: the f in f[x, y]
Length the number of elements in an expression
    
Part (..[[..]]) numbered part of an expression
Level parts specified by a given number of indices
Depth the maximum number of indices to specify any part
    
Symbol  ▪ Integer  ▪ Real  ▪ String  ▪ ...
    
Expression Sizes & Signatures
LeafCount  ▪ ByteCount  ▪ Hash
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