Billboard { 
      eventIn      MFNode   addChildren
      eventIn      MFNode   removeChildren
      exposedField SFVec3f  axisOfRotation  0 1 0      # (-,)
      exposedField MFNode   children        []
      field        SFVec3f  bboxCenter      0 0 0      # (-,)
      field        SFVec3f  bboxSize        -1 -1 -1   # (0,) or -1,-1,-1
    }