z represents a sum of terms.
Plus has attributes Flat, Orderless and OneIdentity.
The default value for arguments of Plus, as used in x_. patterns, is 0.
] is taken to be 0.
] is x.
0 evaluates to x, but x
0.0 is left unchanged.
Unlike other functions, Plus applies built-in rules before user-defined ones. As a result, it is not possible to make definitions such as 2+2=5.
See the Mathematica book: Section 1.1.1.
See also: Minus, Subtract, AddTo, Increment.
This is a fast way to add up the elements of a list.
This shows how a sum is represented internally.
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