• x + y + z represents a sum of terms.
• The default value for arguments of Plus, as used in x_. patterns, is 0.
• Plus[ ] is taken to be 0.
• Plus[x] is x.
• 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.
• New in Version 1; modified in 3.