The contours must be simple closed curves. A violation of this rule occurs if the shape drawn is not a simple closed curve.
The user will be informed that they have drawn a shape that breaks the rule.
Snap To Constrain
The user will be able to select a tool to draw shapes that break the rule but once a shape has been drawn it will be removed if it breaks the rule.
The user will not be able to select a tool that can break the rule, in the event of a tool that can draw shapes that can draw shapes that can both can and can-not break it it will fall back on the Snap To Constraint.