basic:for is mutable, test with formut.bas

assets/formut.bas

@@ -0,0 +1,13 @@
+1 k=0
+2 goto 100
+20 k=1
+30 a=10
+40 return
+100 for a=5 to 1 step -1
+110 print a
+120 if a==3 and k==0 then gosub 20
+130 next
+140 print "=="
+150 print a
+1000 rem expected output according to gw-basic:
+1001 rem 5 4 3 9 8 7 6 5 4 3 2 1 == 0