diff --git a/eval.c b/eval.c
index e3a7ec6..c2d0997 100644
--- a/eval.c
+++ b/eval.c
@@ -63,15 +63,20 @@ int eval_node(node, depth, pvals, pops, ncalcs)
                 res=val1+val2;
                 break;
             case Mul:
-                res=val1*val2;
+                if (val1 > 1 && val2 > 1) /* we avoid "x*1" */
+                    res=val1*val2;
                 break;
             case Sub:
                 if (val1 > val2)
                     res=val1-val2;
+                if (res == val2) /* we already got this value in tree */
+                    res=-1;
                 break;
             case Div:
-                if (val1 > val2 && (val1 % val2 == 0))
+                if (val1 >= val2 && val2 != 1 && (val1 % val2 == 0))
                     res=val1/val2;
+                if (res == val2) /* we already got this value in tree */
+                    res=-1;
                 break;
             case Nop:
             case End: