Execution monitoring
====================

A small set of the default productions print messages at run time, to inform
the user of useful details of execution.  These productions use the {write}
function, and the text they produce is not incorporated in chunks.

Currently, three types of information are printed:

     
  1. Goal success and failure.
     
     A goal succeeds when the state in the subgoal is augmented with one of:
     ^partial-success, ^require-success, or ^success.  A goal fails when it
     is augmented with ^partial-failure, ^prohibit-failure, or ^failure.
     Section See Computing symbolic evaluations describes use of these
     symbols as state augmentations.
     
  2. Values of objects evaluated in the selection space.
     
     When an evaluation object is augmented with either ^symbolic-value or
     ^numeric-value, the value of that attribute is printed.
     
  3. Attribute impasses.
     
     When an impasse object for a non-context slot enters working memory, a
     message describing its structure is printed.

These monitoring productions can be disabled by augmenting the top goal with
^verbose false.