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Chapter 47
`trace.ss`: Tracing Top-level Procedure Calls

To load: `(require (lib "trace.ss"))`

This library mimics the tracing facility available in Chez SchemeTM.

`(trace``` `variable``` `···``)` SYNTAX

Each `variable` must be bound to a procedure in the environment of the `trace` expression. Each `variable` is `set!`ed to a new procedure that traces procedure calls and returns by printing the arguments and results of the call. If multiple values are returned, each value is displayed starting on a separate line.

When traced procedures invoke each other, nested invocations are shown by printing a nesting prefix. If the nesting depth grows to ten and beyond, a number is printed to show the actual nesting depth.

The `trace` form can be used on a variable that is already traced. In this case, assuming that the variable's value has not been changed, `trace` has no effect. If the variable has been changed to a different procedure, then a new trace is installed.

Tracing respects tail calls to preserve loops, but its effect may be visible through continuation marks. When a call to a traced procedure occurs in tail position with respect to a previous traced call, then the tailness of the call is preserved (and the result of the call is not printed for the tail call, because the same result will be printed for an enclosing call). Otherwise, however, the body of a traced procedure is not evaluated in tail position with respect to a call to the procedure.

The value of a `trace` expression is the list of names (as symbols) specified for tracing.

`(untrace``` `variable``` `···``)` SYNTAX

Undoes the effects of the `trace` form for each `variable`, `set!`ing each `variable` back to the untraced procedure, but only if the current value of `variable` is a traced procedure. If the current value of a `variable` is not a procedure installed by `trace`, then the variable is not changed.

The value of an `untrace` expression is the list of names (as symbols) restored to their untraced definitions.