Expression

All expressions can use function call notation.

Evaluate:

求（方程式，公式，函数）的数值.
Expression can be evaluate differently in different Environment.

Evaluation procedure for call expressions:

1. Evaluate the operator and then the operand subexpressions .
2. Apply the function that is the value of the operator subexpression to the arguments that are the values of the operand subexpression.

Procedure for calling/applying user-defined functions:

1. Add a local frame, titled with the <name> of the function being called.
2. Copy the parent of the function to the local frame: [parent=<label>]
3. Bind the <formal parameters> to the arguments in the local frame.
4. Execute the body of the function in the environment that starts with the local frame.

To evaluate a dot expression:

<expression>. <name>

1. Evaluate the <expression> to the left of the dot, which yields the object of the dot expression.
2. <name> is matched against the instance attributes of that object; if an attribute with that name exists, its value is returned.
3. If not, <name> is looked up in the class, which yields a class attribute value.
4. That value is returned unless it is a function, in which case a bound method is returned instead.