4. The method of claim 1 wherein the step of editing a mathematical expression in the mathematical document comprises the step of adding a mathematical expression to the mathematical document.
4. The method of claim 1 wherein the step of editing a mathematical expression in the mathematical document comprises the step of adding a mathematical expression to the mathematical document. 5. The method of claim 4 further comprising the step of adding a node to the directed acyclic graph each time an expression is added to the mathematical document.