UFile (package = ) [public final class AssertionKt {...]
    UClass (name = AssertionKt) [public final class AssertionKt {...}]
        UMethod (name = foo) [public static final fun foo() : java.lang.String {...}]
            UBlockExpression [{...}] : PsiType:Void
                UDeclarationsExpression [var s: java.lang.String = "Not Null"]
                    ULocalVariable (name = s) [var s: java.lang.String = "Not Null"]
                        ULiteralExpression (value = "Not Null") ["Not Null"] : PsiType:String
                UReturnExpression [return s!!] : PsiType:Void
                    UPostfixExpression (operator = !!) [s!!] : PsiType:String
                        USimpleNameReferenceExpression (identifier = s) [s] : PsiType:String
