1. A method for computing a numerical solution to an equation system using a spreadsheet software application, the method comprising: A) displaying and operating a tabular datasheet by executing the spreadsheet software application on a computer device, the tabular datasheet having a plurality of cells each being designated with a column identifier and a row identifier, the cells being configured to receive input and display output; B) receiving, from a first group of the cells, one or more formula expressions representing the equation system and, from a second group of the cells, one or more variables of the formula expressions; C) receiving, from a third group of the cells, a solver function of the spreadsheet software application having one or more first input arguments, the first input arguments at least referencing the formula expressions in the first group of the cells, the variables in the second group of the cells and, any parameters of the equation system; D) computing the solver function to obtain the solution and displaying the solution in the third group of the cells; wherein computing the solver function comprises: D1) constructing, for each of the formula expressions, an evaluable tree-structured graph of relational nodes containing an equivalent sequence of evaluable sub-expressions of said formula expressions; D2) evaluating, by a graph evaluator, the relational nodes in an order of their interdependence to obtain values of the relational nodes based on supplied values of the variables of the formula expressions, and aggregating the values of the relational nodes to obtain the values of the formula expressions in accordance with the supplied values of the variables of the formula expressions; D3) using an underlining algorithm of the solver function to compute the solution in accordance with the formula expressions in the first group of the cells, the variables in the second group of the cells and, the parameters; wherein the underlining algorithm of the solver function utilizes the graph evaluator to evaluate the formula expressions for values of the variables thereby computing the solution.