How to use axlAirGap() to get minimum spacing between two cline segments

Hi Sir,

 I want to get the minimum spacing between two clinesegs(line to line or arc to arc), which were in different layers, such as one segment was in ETCH/layer_3 and the other was in ETCH/layer4. When I used the command axlAirGap(), sometimes it could return the airgap data, but sometimes it returned nil.

Thanks

Bessy