# File rbSnack/tkSnackUtil.rb, line 495
   def getXfor( *indxs )
     raise "unitialized world frame" if @worldFrame.type!=Array or @worldFrame[2]==nil or @worldFrame[0]==nil
     rtv = *indxs.collect{ |i|  screenX2World @boxes[i][0]}
     rtv=rtv[0] if rtv.length==1
     rtv
   end