diff --git a/src/rpn-branch.h b/src/rpn-branch.h
index fd116a7..e951d30 100644
--- a/src/rpn-branch.h
+++ b/src/rpn-branch.h
@@ -3,8 +3,8 @@
 int rpn_if(branch& myobj)
 {
     // myobj.arg1 = 'if' condition evaluation value
-    MIN_ARGUMENTS_RET(1, -1);
-    ARG_MUST_BE_OF_TYPE_RET(0, cmd_number, -1);
+    MIN_ARGUMENTS_RET(1, -(int)ret_runtime_error);
+    ARG_MUST_BE_OF_TYPE_RET(0, cmd_number, -(int)ret_runtime_error);
 
     if (mpfr_cmp_si(((number*)_stack->get_obj(0))->_value.mpfr, 0UL) != 0)
         myobj.arg1 = 1;
@@ -51,9 +51,9 @@ int rpn_start(branch& myobj)
 {
     int ret = -1;
 
-    MIN_ARGUMENTS_RET(2, 1);
-    ARG_MUST_BE_OF_TYPE_RET(0, cmd_number, -1);
-    ARG_MUST_BE_OF_TYPE_RET(1, cmd_number, -1);
+    MIN_ARGUMENTS_RET(2, -(int)ret_runtime_error);
+    ARG_MUST_BE_OF_TYPE_RET(0, cmd_number, -(int)ret_runtime_error);
+    ARG_MUST_BE_OF_TYPE_RET(1, cmd_number, -(int)ret_runtime_error);
 
     // farg2 = last value of start command
     stack::copy_and_push_back(*_stack, _stack->size()-1, _branch_stack);
@@ -79,9 +79,9 @@ int rpn_for(branch& myobj)
 {
     int ret;
 
-    MIN_ARGUMENTS_RET(2, 1);
-    ARG_MUST_BE_OF_TYPE_RET(0, cmd_number, -1);
-    ARG_MUST_BE_OF_TYPE_RET(1, cmd_number, -1);
+    MIN_ARGUMENTS_RET(2, -(int)ret_runtime_error);
+    ARG_MUST_BE_OF_TYPE_RET(0, cmd_number, -(int)ret_runtime_error);
+    ARG_MUST_BE_OF_TYPE_RET(1, cmd_number, -(int)ret_runtime_error);
 
     symbol* sym = ((symbol*)seq_obj(myobj.arg1));
 
@@ -96,11 +96,21 @@ int rpn_for(branch& myobj)
     myobj.farg1 = (number*)_branch_stack.back();
     _stack->pop_back();
 
-    // store symbol with first value
-    _local_heap.add(sym->_value, myobj.farg1, myobj.farg1->size());
-    (void)_stack->pop_back();
+    // test value
+    if (myobj.farg1->_value > myobj.farg2->_value)
+        // last boundary lower than first boundary
+        // -> next command shall be after 'next'
+        // arg2 holds index of 'next'
+        ret = myobj.arg2 + 1;
+    else
+    {
+        // store symbol with first value
+        _local_heap.add(sym->_value, myobj.farg1, myobj.farg1->size());
+        (void)_stack->pop_back();
+        ret = myobj.arg1 + 1;
+    }
 
-    return myobj.arg1 + 1;
+    return ret;
 }
 
 int rpn_next(branch& myobj)
@@ -150,108 +160,59 @@ int rpn_next(branch& myobj)
 
 int rpn_step(branch& myobj)
 {
-static int count = 0;
-    // arg1 = index of start or for command in program
-    // farg1 = current count
-    branch* start_or_for = (branch*)seq_obj(myobj.arg1);
-    if (! myobj.arg_bool)
-    {
-        myobj.arg_bool = true;
-        myobj.farg1 = start_or_for->farg1;
-    }
+    int ret;
+    MIN_ARGUMENTS_RET(1, -(int)ret_runtime_error);
+    ARG_MUST_BE_OF_TYPE_RET(0, cmd_number, -(int)ret_runtime_error);
 
-    // increment then test
     number* step = (number*)_stack->pop_back();
-    mpfr_add(myobj.farg1->_value.mpfr, myobj.farg1->_value.mpfr, step->_value.mpfr, MPFR_DEF_RND);
 
-cout<<"step=";step->show(cout);cout<<endl;
-return -1;
-
-    // for command: increment symbol too
-    if (start_or_for->arg1 != -1)
-    {
-        object* obj;
-        unsigned int size;
-        symbol* var = (symbol*)seq_obj(start_or_for->arg1);
-
-        // increase symbol variable
-        _local_heap.replace_value(string(var->_value), myobj.farg1, myobj.farg1->size());
-    }
-
-    //test value
-    bool step_positive = mpfr_cmp_d(step->_value.mpfr, 0.0)>0;
-
-cout<<"(count="<<count;")\n";
-if (count++>10)
-return -1;
-
-    if ((step_positive && (myobj.farg1 > start_or_for->farg2))
-       || ((! step_positive) && (myobj.farg1 < start_or_for->farg2)))
-    {
-        // end of loop
-        myobj.arg_bool = false;// init again next time
-        _branch_stack.pop_back();
-        _branch_stack.pop_back();
-        return -1;
-    }
+    // end of loop if step is negative or zero
+    if (mpfr_cmp_d(step->_value.mpfr, 0.0)<=0)
+        ret = -1;
     else
     {
-        // for command: next instruction will be after symbol variable
-        if (start_or_for->arg1 != -1)
-            return start_or_for->arg1 + 1;
-        // start command: next instruction will be after start command
-        else
-            return myobj.arg1 + 1;
-    }
-//TODO
-#if 0
-    // arg1 = index of start or for command in program
-    // farg1 = current count
-    number* step = _stack.pop_back();
-
-    branch* start_or_for = (branch*)seq_obj(myobj.arg1);
-    if (! myobj.arg_bool)
-    {
-        myobj.arg_bool = true;
-        myobj.farg1 = start_or_for->farg1;
-    }
-
-    // increment then test
-    mpfr_add(&myobj.farg1.mpfr, &myobj.farg1.mpfr, &step.mpfr, MPFR_DEF_RND);
-
-    // for command: increment symbol too
-    if (start_or_for->arg1 != -1)
-    {
-        void* obj;
-        unsigned int size;
-        int type;
-        symbol* var = (symbol*)seq_obj(start_or_for->arg1);
-
-        // check symbol variable is a number, then increase
-        if (_local_heap.get(string(var->_value), obj, size, type) && (type == cmd_number))
+        // arg1 = index of start or for command in program
+        // farg1 = current count
+        branch* start_or_for = (branch*)seq_obj(myobj.arg1);
+        if (! myobj.arg_bool)
         {
-            ((number*)obj)->_value = myobj.farg1;
+            myobj.arg_bool = true;
+            myobj.farg1 = start_or_for->farg1;
+        }
+
+        // increment then test
+        mpfr_add(myobj.farg1->_value.mpfr, myobj.farg1->_value.mpfr, step->_value.mpfr, MPFR_DEF_RND);
+
+        // for command: increment symbol too
+        if (start_or_for->arg1 != -1)
+        {
+            object* obj;
+            unsigned int size;
+            symbol* var = (symbol*)seq_obj(start_or_for->arg1);
+
+            // increase symbol variable
+            _local_heap.replace_value(string(var->_value), myobj.farg1, myobj.farg1->size());
+        }
+
+        // test loop value is out of range
+        if (myobj.farg1->_value > start_or_for->farg2->_value)
+        {
+            // end of loop
+            myobj.arg_bool = false;// init again next time
+            _branch_stack.pop_back();
+            _branch_stack.pop_back();
+            ret = -1;
+        }
+        else
+        {
+            // for command: next instruction will be after symbol variable
+            if (start_or_for->arg1 != -1)
+                ret = start_or_for->arg1 + 1;
+            // start command: next instruction will be after start command
+            else
+                ret = myobj.arg1 + 1;
         }
     }
-
-    //test value
-    if (((step>0) && (myobj.farg1 > start_or_for->farg2))
-       || ((step<0) && (myobj.farg1 < start_or_for->farg2)))
-    {
-        // end of loop
-        myobj.arg_bool = false;// init again next time
-        return -1;
-    }
-    else
-    {
-        // for command: next instruction will be after symbol variable
-        if (start_or_for->arg1 != -1)
-            return start_or_for->arg1 + 1;
-        // start command: next instruction will be after start command
-        else
-            return myobj.arg1 + 1;
-    }
-#endif
-
-    return -1;
+    
+    return ret;
 }
diff --git a/src/rpn.cpp b/src/rpn.cpp
index 2ce830f..7570745 100644
--- a/src/rpn.cpp
+++ b/src/rpn.cpp
@@ -75,12 +75,13 @@ typedef enum {
     ret_nop,
     ret_syntax,
     ret_div_by_zero,
+    ret_runtime_error,
     ret_max
 } ret_value;
 
 const char* ret_value_string[ret_max] = {
     "ok", "unknown command", "missing operand", "bad operand type", "out of range", "unknown variable", "internal error, aborting",
-    "deadly", "goodbye", "not implemented", "no operation", "syntax", "division by zero"
+    "deadly", "goodbye", "not implemented", "no operation", "syntax error", "division by zero", "runtime error"
 };
 
 typedef enum {
@@ -491,10 +492,20 @@ public:
                 // call matching function
                 branch* b = (branch*)seq_obj(i);
                 int next_cmd = (this->*(b->_fn))(*b);
-                if (next_cmd == -1)
-                    i++;
-                else
-                    i = next_cmd;
+                switch (next_cmd)
+                {
+                    case -1:
+                        i++; // meaning 'next command'
+                        break;
+                    case -(int)ret_runtime_error:
+                        // error: show it
+                        (void)show_error(_err, _err_context);
+                        go_out = true;// end of run
+                        break;
+                    default:
+                        i = next_cmd;// new direction
+                        break;
+                }
             }
 
             // not a command, but a stack entry, manage it
@@ -660,8 +671,8 @@ public:
                         show_syntax_error("missing start or for before next");
                         return ret_syntax;
                     }
-                    k->arg1 = vstartindex[vstartindex.size() - 1];// fill 'next' branch1 = 'start' index
-                    ((branch*)seq_obj(vstartindex[vstartindex.size() - 1]))->arg2 = i;// fill 'for' or 'start' arg2 = 'next' index
+                    k->arg1 = vstartindex[vstartindex.size() - 1]; // 'next' arg1 = 'start' index
+                    ((branch*)seq_obj(vstartindex[vstartindex.size() - 1]))->arg2 = i; // 'for' or 'start' arg2 = 'next' index
                     vstartindex.pop_back();
                 }
                 else if (compare_branch(k, "step", 4))
@@ -673,6 +684,7 @@ public:
                         return ret_syntax;
                     }
                     k->arg1 = vstartindex[vstartindex.size() - 1];// fill 'step' branch1 = 'start' index
+                    ((branch*)seq_obj(vstartindex[vstartindex.size() - 1]))->arg2 = i; // 'for' or 'start' arg2 = 'next' index
                     vstartindex.pop_back();
                 }
                 else if (compare_branch(k, "->", 2))
@@ -696,28 +708,46 @@ public:
         return ret_ok;
     }
 
-    static ret_value show_error(ret_value err, string& context)
+    ret_value show_error()
     {
-        cerr<<context<<": "<<ret_value_string[err]<<endl;
-        switch(err)
+        ret_value ret;
+
+        // show last recorded error
+        cerr<<ret_value_string[_err]<<"("<<_err<<"): "<<_err_context<<endl;
+        switch(_err)
         {
             case ret_internal:
             case ret_deadly:
-                return ret_deadly;
+                ret = ret_deadly;
             default:
-                return ret_ok;
+                ret = ret_ok;
         }
+        
+        return ret;
     }
 
-    static ret_value show_error(ret_value err, char* context)
+    ret_value show_error(ret_value err, string& context)
     {
-        string context_string(context);
-        return show_error(err, context_string);
+        // record error
+        _err = err;
+        _err_context = context;
+        return show_error();
     }
 
-    static void show_syntax_error(const char* context)
+    ret_value show_error(ret_value err, const char* context)
     {
-        cerr<<"syntax error: "<<context<<endl;
+        // record error
+        _err = err;
+        _err_context = context;
+        return show_error();
+    }
+
+    void show_syntax_error(const char* context)
+    {
+        // record error
+        _err = ret_syntax;
+        _err_context = context;
+        (void)show_error();
     }
 
     ret_value get_err(void) { return _err; }