public struct DictPair {
            public string from;
            public string to;
            public int    fromLen;
            public int    toLen;

            public DictPair(string from, string to) {
                this.from = from;
                this.to   = to;
                fromLen   = from.Length;
                toLen     = to.Length;
            }
        }

        private void ReplaceWord(RichTextBox rtb, DictPair w) {
            var src     = w.@from;
            var dest    = w.to;
            var srcLen  = w.fromLen;
            var destLen = w.toLen;

            var start = 0;
            var index = rtb.Find(src, start, rtb.Rtf.Length, RichTextBoxFinds.WholeWord | RichTextBoxFinds.MatchCase);
            while (index >= 0) {
                rtb.Select(index, srcLen);
                rtb.SelectedText = dest;
                rtb.Select(index, destLen);
                tStruct.AddArea(new TextArea(index, destLen), srcLen);
                //rtb.SelectionColor =  Color.Green;
                start += index + destLen;
                if (start < rtb.Text.Length)
                    index = rtb.Find(src, start, rtb.Rtf.Length, RichTextBoxFinds.WholeWord | RichTextBoxFinds.MatchCase);
                else
                    index = -1;
            }
        }