Further constrain types of conformance operations

interface/HUTN/includes/conformance_scd.alh

@@ -1,8 +1,8 @@
-Element function is_direct_instance(a: Element, b: Element, c: Element)
-Element function is_nominal_instance(a: Element, b: Element, c: Element)
-Element function is_nominal_subtype(a: Element, b: Element, c: Element, d: Element)
-Element function is_structural_subtype(a: Element, b: Element)
-Element function is_structural_instance(a: Element, b: Element, c: Element)
+Boolean function is_direct_instance(a: Element, b: Element, c: Element)
+Boolean function is_nominal_instance(a: Element, b: Element, c: Element)
+Boolean function is_nominal_subtype(a: Element, b: Element, c: Element, d: Element)
+Boolean function is_structural_subtype(a: Element, b: Element)
+Boolean function is_structural_instance(a: Element, b: Element, c: Element)
 Element function conformance_scd(a: Element)
 Element function retype(a: Element, b: Element, c: Element, d: Element)
 Element function add_to_model(a: Element, b: String, c: Element)