fix bug in repeated function calls

ast/eval.go

@@ -43,17 +43,16 @@ func (in *Token) Eval(funcs FuncTable, vars VarTable, cnvtUndefVars bool) *Token
 
     switch in.Tag {
     case BOOL, NUMBER, STRING:
-        res = in
+        res = in.Copy()
 
     case SYMBOL:
         res = GetVar(in.Value(), vars)
         if res == nil {
-            res = in
+            res = in.Copy()
 
             if GetFunction(in.Value(), funcs) == nil {
                 if cnvtUndefVars {
-                    in.Tag = STRING
-                    res = in
+                    res.Tag = STRING
                     break
                 }
 
@@ -63,19 +62,17 @@ func (in *Token) Eval(funcs FuncTable, vars VarTable, cnvtUndefVars bool) *Token
                 return nil
             }
         }
-        res.Next = in.Next
-
 
     case LIST:
         inner := in.Expand()
         if inner == nil {
-            res = in
+            res = in.Copy()
             break
         }
 
         if inner.Tag != SYMBOL {
             in.Direct(inner.Eval(funcs, vars, cnvtUndefVars))
-            res = in
+            res = in.Copy()
             break
         }