The PDA construction tool requires that one type "epsilon" into the tape column of the transition
creation table if it is desired that the PDA read no input on the transition being created.  On
the other hand, the pop and push entries are simply left blank if no push or pop is required.  The
applet should be changed so that the tape entry is left blank as well if no read is to be done.
This will likely require some special programming so that when the OK button is pushed, the 
builder really does know that an empty transition is being created if all columns are left blank.

This applet could be enhanced in a number of ways:

  - allow for a tree view of the computations
  - allow for certain branches to be stopped by the user
  - allow for certain branches to be selected by the user
  - allow the user to specify a sequence of epsilon and alphabet symbols that
    would take the PDA to an accept state that is then compared for accuracy 
    against the PDA.