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 14.3.1.4 Evaluating Special Syntactic Expressions
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 When a procedure invocation expression is evaluated, the procedure and
 _all_ the argument expressions must be evaluated before the procedure
 can be invoked.  Special syntactic expressions are special because they
 are able to manipulate their arguments in an unevaluated form, and can
 choose whether to evaluate any or all of the argument expressions.
 
    Why is this needed?  Consider a program fragment that asks the user
 whether or not to delete a file, and then deletes the file if the user
 answers yes.
 
      (if (string=? (read-answer "Should I delete this file?")
                    "yes")
          (delete-file file))
 
    If the outermost `(if ...)' expression here was a procedure
 invocation expression, the expression `(delete-file file)', whose side
 effect is to actually delete a file, would already have been evaluated
 before the `if' procedure even got invoked!  Clearly this is no use --
 the whole point of an `if' expression is that the "consequent"
 expression is only evaluated if the condition of the `if' expression is
 "true".
 
    Therefore `if' must be special syntax, not a procedure.  Other
 special syntaxes that we have already met are `define', `set!' and
 `lambda'.  `define' and `set!' are syntax because they need to know the
 variable _name_ that is given as the first argument in a `define' or
 `set!' expression, not that variable's value.  `lambda' is syntax
 because it does not immediately evaluate the expressions that define
 the procedure body; instead it creates a procedure object that
 incorporates these expressions so that they can be evaluated in the
 future, when that procedure is invoked.
 
    The rules for evaluating each special syntactic expression are
 specified individually for each special syntax.  For a summary of
 standard special syntax, see  Syntax Summary.
 

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