recently there was discussion about bulge/fillet http://www.theswamp.org/index.php?topic=55931.0
look at ripp-bulge -- Radius-InterPoint-startPoint-endPoint
... if center is unknown, but known = radius , vertex (Intersection point) , p1 , p2 (note: vertex =/= center)returns list (start-bulge-end)like fillet method (can be applied trans on 3Dpoly as well)