Fix scoping with mutable variables

test/tests/scope checkpoint mutable ter.ans

@@ -0,0 +1,42 @@
+:x = [99]
+
+:l = [1,x]
+
+:n = 0
+
+$ f(t)
+	~ t!insert(len(l)+1)
+
+	f1: {l} {t}
+
+	CHECK
+	§ c
+
+	~ n < 1
+		REC
+
+		~ n += 1
+		~ f(t)
+
+		END REC
+
+	~ x!insert(12)
+
+	f2: {l}
+
+	CHECK 2
+	§ d
+
+	~ t!insert(len(t)+1)
+
+	~ t(2)!insert(len(l)+1)
+
+	f3: {l} {t}
+
+~ f(l)
+
+FINAL
+
+l: {l}
+
+x: {x}