ImageSegment discoverActiveClasses.
is cool and it lets you (by swapping method dict behind your back) knowing
which classes have been involved in a certain action.

as follow
Now ImageSegment discoverActiveClasses.
      -- do something typical --
  ImageSegment activeClasses size printString , ' classes were active out
of ' ,
    Smalltalk allClasses size printString.

Now if following
  recoverFromMDFaultWithTrace

you put

Smalltalk at: #MDFaultDict put: Dictionary new.

you get a strange error, tell you that there is syntax error

Stef