enhanced the compiler to prohibit variable declaration after 'do' if it's the head of block expression

t/Makefile.am

@@ -9,6 +9,7 @@ check_SCRIPTS = \
 	va-01.hcl
 
 check_ERRORS = \
+	do-01.err \
 	feed-01.err \
 	var-01.err \
 	var-02.err \

t/Makefile.in

@@ -480,6 +480,7 @@ check_SCRIPTS = \
 	va-01.hcl
 
 check_ERRORS = \
+	do-01.err \
 	feed-01.err \
 	var-01.err \
 	var-02.err \

t/do-01.err

@@ -0,0 +1,3 @@
+## if `do` is not enclosed in `( )`, variable declaration is prohibited
+(do | k | (set k 10))
+do | k | {set k 10;}; ##ERROR: syntax error - variable declaration disallowed