Detecting backward motion represented by a path

A system for processing graphical input, the system comprising:

• means for receiving input specifying a path including a plurality of locations ordered along the path;
• means for specifying a tracking zone for each location in the plurality of locations based on the location and a previous location preceding the location along the path, the tracking zone indicating a direction of the path at the location;
• and
• means for detecting a backward motion between a first and a second location in the path if the first location's tracking zone overlaps with the second location's tracking zone.

A software product tangibly embodied in a machine-readable storage device, the software product comprising instructions operable to cause one or more data processing apparatus to perform operations comprising:

• receiving input specifying a path including a plurality of locations ordered along the path;
• for each location in the plurality of locations, specifying a tracking zone based on the location and a previous location preceding the location along the path, the tracking zone indicating a direction of the path at the location;
• and
• detecting a backward motion between a first and a second location in the path if the first location's tracking zone overlaps with the second location's tracking zone.