20. The system of claim 18 , wherein the walker is further configured to: identify a left-to-right direction when a text part contains at least one character belonging to a left-to-right script but no characters belonging to a right-to-left script; and identify a right-to-left direction when a text part contains at least one character belonging to a right-to-left script but no characters belonging to a left-to-right script.
21. The system of claim 20 , wherein the walker is further configured to identify a neutral direction when a text part contains neither left-to-right nor right-to-left script characters.