<s:source-element1 ...> <property_1 ...> <property_2 ...> <property_3 ...> </s:element1>
rule SourceElement1_to_Target_structure { from s1 : s!source-element1 to t1 : t!target-element1(...), -- a unique element I must reference to in every t2 element t2 : Set(t!target-element2(s, t1)) -- such structure is not allowed; used only to demonstrate the need }