Forms which are lists are used to call a subroutine. The first element of the list is the subroutine which is to be called; all further elements are arguments to be applied to the subroutine.
There are several different types of subroutines available: functions, macros, special forms and autoloads. When the evaluator finds a form which is a list it tries to classify the form into one of these four types. First of all it looks at the first element of the list, if it is a symbol it gets the value from the function slot of the symbol (note that the first element of a list form is never evaluated itself). This value (either the first element or the symbol's function value) is enough to classify the form into one of the four types.
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