A methods, apparatus and product for explaining partially illegal combinations in combinatorial models. The method comprising: obtaining a combinatorial model defining a legal test space, the combinatorial model comprising a set of attributes, a respective domain for each attribute defining possible values for the attribute, and a set of restrictions, wherein the restrictions define a combination of values of the attributes that are illegal and are excluded from the legal test case； obtaining a partially illegal combination defining value assignments to a portion of the attributes； automatically identifying an extension of the partially illegal combination, wherein the extension is excluded from the legal test space, wherein the extension can be modified to become legal by changing a portion of the value assignments defined by the partially illegal combination； and outputting the extension.
