5. The method of claim 4 , further comprising reducing the number of said graphical renderings displayed concurrently by displaying one rendering for all nodes belonging to the same node group.
4. The method of claim 1 , further comprising displaying a graphical rendering of one or more of said expression hierarchies concurrently within a view, the graphical rendering including text, shapes, and lines, the text including the contents of the formulas represented, by the nodes, the shapes representing the nodes, and the lines representing the edges connecting the nodes within said expression hierarchies.