Makes the first selected circle/arc have the same radius as the second circle/arc.
- Start with two circles/arcs. Activate the Equal Radius constraint and select the arc whose radius you want to change.
- Select the arc whose radius you want to assign to the first arc.
The first arc's radius changes to match the second. The equal constraint (an equal sign) appears. 
- If you edit either arc, the equal radius constraint will be maintained.
