Automatic edge-breaking?

[timwarner]

I'm trying to use Ruby .add_line to add an edge to a surface broken into an upper and lower face by a curve. When I use add_line to add a single edge, top to bottom, the edge isn't automatically broken by Sketchup, which I thought was the default (Sketchup.break_edges is certainly "true"). What's the solution? (It's a bit tough to figure out where the new edge would cross the curve, so difficult to add two edges separately.)