predictive explorer: report unknow boundaries

In #173 sliders (now textboxes) become aware of a few functions that can be applied to the parameters. Providing a truly general solution will probably be too hard, and it may require a different approach to parse the function. Something easier may be to detect 3 categories, no function has been applied, a function/operation is applied and we can solve it, and a function/operation is applied but we don't have a clue what is going on, for this later case we could return something like "(unk, unk)", signaling that the function is not able to guess the boundaries of the parameter.

Start by reading issue #173 and the predictive explorer code that handles slider/textbox parameter boundaries. Trace how functions are parsed and how boundaries are reported; done means no-function, solvable-function, and unknown-function cases are distinguished, with the last returning "(unk, unk)" without breaking the other cases.

Written by the indexing model from the issue text.