#### 4.1.3 Procedure calls

— syntax: (<operator> <operand1> ...)

> A procedure call is written by simply enclosing in parentheses expressions for the procedure to be called and the arguments to be passed to it. The operator and operand expressions are evaluated (in an unspecified order) and the resulting procedure is passed the resulting arguments.

>          (+ 3 4)                                ==>  7
>          ((if #f + *) 3 4)                      ==>  12

> A number of procedures are available as the values of variables in the initial environment; for example, the addition and multiplication procedures in the above examples are the values of the variables ‘`+`’ and ‘`*`’. New procedures are created by evaluating lambda expressions (see section see Procedures).

> Procedure calls may return any number of values (see `values` in section see Control features). With the exception of ‘`values`’ the procedures available in the initial environment return one value or, for procedures such as ‘`apply`’, pass on the values returned by a call to one of their arguments.

> Procedure calls are also called combinations.

> > Note: In contrast to other dialects of Lisp, the order of evaluation is unspecified, and the operator expression and the operand expressions are always evaluated with the same evaluation rules.

> > Note: Although the order of evaluation is otherwise unspecified, the effect of any concurrent evaluation of the operator and operand expressions is constrained to be consistent with some sequential order of evaluation. The order of evaluation may be chosen differently for each procedure call.

> > Note: In many dialects of Lisp, the empty combination, `()`, is a legitimate expression. In Scheme, combinations must have at least one subexpression, so `()` is not a syntactically valid expression.