1. In a computer system having a plurality of processor cells each of which can contain a formula, a method comprising: writing a formula in each cell, the formula containing one or more variables; coupling each of the cells to a global memory; storing variable values in the global memory; importing, at the beginning of a frame, values for the variables into the cells from the global memory; exporting, at the end of a frame, new variable values computed by the cells using said imported values, from those cells to the global memory; defining a first level matrix of said cells; defining at least one other level matrix of said cells, this level matrix being contained in a cell of the first level; and defining a lowest level matrix of said cells each of which contains only code.