wtf is sofs useful for

gistfile1.txt

1> ParentChildRel = sofs:relation([{michel,gertrude},{robert,mickael},{brigitte,gertrude},{brigitte,sophia}], [{parent, child}]).
{'Set',[{brigitte,gertrude},
        {brigitte,sophia},
        {michel,gertrude},
        {robert,mickael}],
       {parent,child}}
2> ParentChildrenRel = sofs:relation_to_family(ParentChildRel).
{'Set',[{brigitte,[gertrude,sophia]},
        {michel,[gertrude]},
        {robert,[mickael]}],
       {parent,[child]}}
3> sofs:to_external(ParentChildrenRel).
[{brigitte,[gertrude,sophia]},
 {michel,[gertrude]},
 {robert,[mickael]}]