1. A computer controlled method for determining a set of obligatory parent facts of at least one edge in a chart comprising: a) receiving the chart, the chart having a plurality of chart edges wherein at least one edge has at least one fact associated therewith; b) determining a set of obligatory facts for at least one one of the plurality of chart edges; and c) selecting the set of obligatory parent facts of a given edge those facts which are in the set of obligatory facts of at least one of the plurality of chart edges in at least one parent path of the given edge.