Although there are many TCAD systems which successfully use Tcl/Tk [48] [51] as task level interpreter and user interface (see, e.g., [7], [9], [12], [83]), VISTA's task level environment is predominantly based on XLISP [84], a comprehensible and compact LISP interpreter. The highly portable C source code of this public-domain product is freely available. It can be extended and customized for TCAD purposes by both adding C-coded primitives and by loading LISP code at run time.
The appropriateness of this choice is also confirmed by several other remarkable, LISP-based implementations of task level environments in related fields. Examples are the well-known GNU Emacs [81] text editor which uses LISP as extension and top-level implementation language, the generic CAD system AutoCAD [82] which derives much of its success from third-party applications implemented in the SCHEME-like extension language AutoLISP, or Winterp [77] (``Widget Interpreter''), an experimental user interface prototyping environment which is part of the MIT X11 distribution.
Other integrated CAD systems which allow for the definition of complex, data-driven control flows often use LISP as major implementation language [49] [50].