ly( 'shift', bb, xdel, ydel )
 Get center of bbs.
   cen = bbApply( 'getCenter', bb )
 Get bb at intersection of bb1 and bb2 (may be empty).
   bb = bbApply( 'intersect', bb1, bb2 )
 Get bb that is union of bb1 and bb2 (smallest bb containing both).
   bb = bbApply( 'union', bb1, bb2 )
 Resize the bbs (without moving their centers).
   bb = bbApply( 'resize', bb, hr, wr, [ar] )
 Fix bb aspect ratios (without moving the bb centers).
   bbr = bbApply( 'squarify', bb, flag, [ar] )
 Draw single or mu