🚀 go-pugleaf

Although a number of wysiwyg editors can display math symbols, the original
poster asked about EXTENSIBLE wysiwyg systems.  Thus, for example, on the
Xerox star one can easily create and display on the screen summations or
integrals with bounds in the proper places by selecting the "sigma" or
"Integral" template in equation mode.  Of course it's recursive so you
can create things like:
		  infinity
		SIGMA f(k)
		  k=1
	INTEGRAL		y(x) dx
		37

What you can NOT do with that editor, or with any other with which I'm
familiar, is create your own notation.  Thus, suppose you want to have RHO
behave like SIGMA, or want to create a notation that uses overbars to 
indicate binding, e.g.:

		          ___
		 ________/___\
		/	/    \\
	FORALL a EXISTS b (f(a,b))

Is there any commercially available product?