8.5 Standard Procedures
This section describes the expression language's built-in procedures.  The initial (or top-level) environment starts out with a number of variables bound to useful values, most of which are primitive procedures that manipulate data.  For example, the variable abs is bound to a procedure of one argument that computes the absolute value of a number, and the variable + is bound to a procedure that computes sums.
It shall be an error for a procedure to be passed an argument of a type that it is not specified to handle.