• Evaluate[expr] 使 expr 被赋值，即使它作为一个具有指定它不应被赋值的属性的函数的参数出现.
•例如: Hold[Evaluate[1 + 1]]
Evaluate causes the list of functions to be constructed immediately, so that Plot will know that it needs to plot more than one curve.
Evaluate is needed because Plot has the HoldAll attribute.
If Evaluate is omitted, this example will generate an error, since Plot expects the first argument to evaluate to a number rather than to a list of numbers.
Evaluate is not necessary here, since Factor does not hold its arguments unevaluated.
Evaluate has no effect unless the enclosing expression is evaluated.
This shows one way to put an evaluated expression into a position that would otherwise be held unevaluated.