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Atomic Elements of Expressions
All expressions in Mathematica are ultimately built from a small number of distinct types of atomic elements.
AtomQ test whether an expression is an atom
Head find the head of any expression, including an atom
    
Symbol the head of a symbol; create a symbol from a name
SymbolName  ▪ Context  ▪ Remove  ▪ Names  ▪ NameQ  ▪ ...
    
String the head of a string
StringQ  ▪ Characters  ▪ ToCharacterCode  ▪ StringLength  ▪ ToExpression  ▪ ...
    
Integer  ▪ Real  ▪ Rational  ▪ Complex
NumberQ test for a number of any type
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