func (c *ComposedHandler) callMap(h bh.Handler, msg bh.Msg, ctx bh.MapContext) (
cells bh.MappedCells, err error) {
defer func() {
if r := recover(); r != nil {
err = errors.New(fmt.Sprintf("%v", r))
}
}()
cells = h.Map(msg, ctx)
if cells == nil {
err = fmt.Errorf("%#v drops the msg", h)
}
return
}
