some compiler enhancements

t/do-02.err

@@ -0,0 +1,8 @@
+## if `do` is not enclosed in `( )`, it supports only the limited number of expressions.
+
+do ; ## this is ok
+
+do 1; ## this is ok
+
+do { set k 10; printf "k=%d\n" k; } ##ERROR: syntax error - more than one expression after do
+   { set k 20; printf "k=%d\n" k; }; 

t/feed-01.err

@@ -0,0 +1,6 @@
+## the left brace opens a list explicitly and doesn't auto-forge a container list.
+## so the semicolon after it is a redundant one. `do {};` would work because the semicolon
+## terminates the auto-forged list of `do` and `{}`.
+{
+	printf "hello, world\n";
+}; ##ERROR: syntax error - unexpected semicolon

t/var-03.err

@@ -0,0 +1,3 @@
+defclass A | a | {
+	| j | ##ERROR: syntax error - variable declaration disallowed in class init scope
+};