Catch browse back beyond start

go/state/state.go

@@ -6,6 +6,13 @@ import (
 	"strings"
 )
 
+type IndexError struct {
+}
+
+func(err *IndexError) Error() string {
+	return fmt.Sprintf("already at first index")
+}
+
 // State holds the command stack, error condition of a unique execution session.
 //
 // It also holds cached values for all results of executed symbols.
@@ -208,7 +215,7 @@ func(st *State) Previous() (uint16, error) {
 		return 0, fmt.Errorf("state root node not yet defined")
 	}
 	if st.sizeIdx == 0 {
-		return 0, fmt.Errorf("already at first index")
+		return 0, &IndexError{} // ("already at first index")
 	}
 	st.sizeIdx -= 1
 	s, idx := st.Where()

go/vm/runner.go

@@ -313,7 +313,18 @@ func(vm *Vm) RunInCmp(b []byte, ctx context.Context) ([]byte, error) {
 	}
 
 	target, _, err = applyTarget([]byte(target), vm.st, vm.ca, ctx)
-	if err != nil {
+	_, ok := err.(*state.IndexError)
+	if ok {
+		_, err = vm.st.ResetFlag(state.FLAG_INMATCH)
+		if err != nil {
+			panic(err)
+		}
+		_, err = vm.st.SetFlag(state.FLAG_READIN)
+		if err != nil {
+			panic(err)
+		}
+		return b, nil
+	} else if err != nil {
 		return b, err
 	}
 	vm.Reset()