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
is not specified to handle.