15. A system as recited in claim 14 wherein said means for determining comprises means for determining that the parsed tree is not changed if said edit changes only a value or an operator of said equation.
15. A system as recited in claim 14 wherein said means for determining comprises means for determining that the parsed tree is not changed if said edit changes only a value or an operator of said equation. 16. A system as recited in claim 15 wherein said means for editing said parsed tree comprises means for substituting a changed value or a changed operator into said structure of said parsed tree.