1. A system for providing alternative computational results in a computer-implemented model, comprising: a plurality of cells each representing a piece of data in the computational model; for each cell, a plurality of formulas indicating alternative values of the respective cell; a plurality of layers arranged to group together formulas from different ones of the plurality of cells; and software for selectively enabling and disabling the plurality of formulas for the plurality of cells on a layer-by-layer basis, wherein the software is operative to produce one value for each of the plurality of cells at a time.