fix: derived properties return unmodifiable collections

src/main/java/spoon/support/reflect/declaration/CtClassImpl.java

@@ -76,7 +76,7 @@ public List<CtAnonymousExecutable> getAnonymousExecutables() {
 				anonymousExecutables.add((CtAnonymousExecutable) typeMember);
 			}
 		}
-		return anonymousExecutables;
+		return Collections.unmodifiableList(anonymousExecutables);
 	}
 
 	@Override
@@ -107,7 +107,7 @@ public Set<CtConstructor<T>> getConstructors() {
 				constructors.add((CtConstructor<T>) typeMember);
 			}
 		}
-		return constructors;
+		return Collections.unmodifiableSet(constructors);
 	}
 
 	@Override

src/main/java/spoon/support/reflect/declaration/CtEnumImpl.java

@@ -132,7 +132,7 @@ public List<CtField<?>> getFields() {
 		List<CtField<?>> result = new ArrayList<>();
 		result.addAll(getEnumValues());
 		result.addAll(super.getFields());
-		return result;
+		return Collections.unmodifiableList(result);
 	}
 
 	@Override

src/main/java/spoon/support/reflect/declaration/CtTypeImpl.java

@@ -285,7 +285,7 @@ public List<CtField<?>> getFields() {
 				fields.add((CtField<?>) typeMember);
 			}
 		}
-		return fields;
+		return Collections.unmodifiableList(fields);
 	}
 
 	@Override
@@ -457,7 +457,7 @@ public Set<CtType<?>> getNestedTypes() {
 				nestedTypes.add((CtType<?>) typeMember);
 			}
 		}
-		return nestedTypes;
+		return Collections.unmodifiableSet(nestedTypes);
 	}
 
 	@Override
@@ -828,7 +828,7 @@ public Set<CtMethod<?>> getMethods() {
 				methods.add((CtMethod<?>) typeMember);
 			}
 		}
-		return methods;
+		return Collections.unmodifiableSet(methods);
 	}
 
 	@Override

src/main/java/spoon/support/reflect/reference/CtTypeReferenceImpl.java

@@ -509,7 +509,7 @@ public CtTypeReference<?> getSuperclass() {
 	public Set<CtTypeReference<?>> getSuperInterfaces() {
 		CtType<?> t = getDeclaration();
 		if (t != null) {
-			return t.getSuperInterfaces();
+			return Collections.unmodifiableSet(t.getSuperInterfaces());
 		} else {
 			Class<?> c = getActualClass();
 			Class<?>[] sis = c.getInterfaces();
@@ -518,7 +518,7 @@ public Set<CtTypeReference<?>> getSuperInterfaces() {
 				for (Class<?> si : sis) {
 					set.add(getFactory().Type().createReference(si));
 				}
-				return set;
+				return Collections.unmodifiableSet(set);
 			}
 		}
 		return Collections.emptySet();