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based on an earlier version of the Wolfram Language.
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Defining Variables and Functions
The symbolic language paradigm of Mathematica takes the concept of variables and functions to a new level. In Mathematica a variable can not only stand for a value, but can also be used purely symbolically. And building on Mathematica's powerful pattern language, "functions" can be defined not just to take arguments, but to transform a pattern with any structure.
x=... set a variable
f[x_]:=... define a function that takes any single argument
Set (=) immediate assignment (right-hand side evaluated immediately)
SetDelayed (:=) delayed assignment (right-hand side evaluated only when used)
Unset (=.) unset a variable
Clear clear a function definition
Module, ... scope local variables
e;e;e (CompoundExpression) execute expressions in sequence
Attributes  ▪ Flat  ▪ Orderless  ▪ Listable  ▪ HoldFirst  ▪ Protected