Abstract: A method, apparatus and product for determining a derived restriction in a combinatorial model. The method comprising: obtaining a combinatorial model defining a test space, the combinatorial model comprising a set of attributes, a respective domain for each attribute, and a set of restrictions, wherein the restrictions define a combination of values of the attributes that are restricted from the test space; and automatically determining a derived restriction that is derived from at least a first restriction and a second restriction, the first and second restrictions are comprised by the set of restrictions, wherein the derived restriction restricts at least one combination of values that is not restricted by the first restriction and at least one combination of values that is not restricted by the second restriction.