Updated Principles (markdown)

Principles.md

@@ -17,10 +17,8 @@ Extracted from her presentation at Camp Smalltalk 50th Anniversary.
 * So an important quality of this kit is that it is always working, always displaying what the model
 represents (_on Programming by Rehearsal_).
 
-* What also makes this kit so interesting is its view that constraints, whether equations to be
-solved or relations to be maintained, can express partial information, can be combined with other
-constraints, can be general purpose in the sense of their properties, are declarative, and can be
-searched and experimented with. So, these represent an important class of base objects we would
-need in a modeling environment (_on ThingLab kit_).
+* What also makes this kit so interesting is its view that constraints, whether equations to be solved or relations to be maintained, can express partial information, can be combined with other constraints, can be general purpose in the sense of their properties, are declarative, and can be searched and experimented with. So, these represent an important class of base objects we would need in a modeling environment (_on ThingLab kit_).
 
 * It would be a mistake to assume that there is just one virtual world of simulation as generality often proves to make things harder for beginners (_on Alternate Reality kit_).
+
+* An important point made by our teaching experience is that, because we expect the learner to extend a model through programming new methods, not just by parameter modification, it is critical that some outcome (_error_) that the learner mistakenly creates does not get explained (_debugger_) using vocabulary outside (_i.e. system class_) of what has been already been introduced (_on Simulation kit_).