What is the _Stepper_?

DrScheme includes an "algebraic stepper," a tool which proceeds through the 
evaluation of a set of definitions and expressions, one step at a time. 
This evaluation shows the user how DrScheme evaluates expressions and 
definitions, and can help in debugging programs.  Currently, the Stepper is 
available in the "Beginner" and "Intermediate" language levels.

How do I use the Stepper?

The Stepper operates on the contents of the frontmost DrScheme window.  A click 
on the "Step" button brings up the stepper window.  The stepper window has 
three boxes; each one is separated by a blue horizontal line.

The first box shows definitions and expressions whose evaluation is 
complete. This box is changed only when another completed evaluation's 
result is added to it.  

The second box shows the current expression.  The region highlighted in 
green is known as the "redex".  You may pronounce this word in any way you 
want.  It is short for "reducible expression," and it is the expression 
which is the next to be simplified.

The third box shows the result of the reduction.  The region highlighted 
in purple is the new expression which is substituted for the green one as 
a result of the reduction.

For more information on how DrScheme selects a "redex" and how the 
expressions are reduced, please see <A>The Beginner Language Semantics</A> 
which formally specify the set of rules governing the language's evaluation.

There are three buttons at the top of the stepper window.  The "Home" 
button returns to the initial state of the evaluation: id est, no 
expressions have yet been evaluated.  The "Previous" button returns to the 
prior step of the evaluation.  The "Next" step shows the next step in the 
evaluation.
