// layouter: voidfunc(Container)
// or func(Container) -> Dimension [preferred size]
// or func(Container) -> Pair(Dimension, Dimension) [preferred, minimum]
static LayoutManager layoutManagerFromFunction(fO layouter) {
  if (layouter cast LayoutManager) ret layouter;
  
  ret new AbstractLayoutManager {
    public void layoutContainer(Container parent) {
      O size = pcallF(layouter, parent);
      if (size cast Dimension)
        preferredSize = size;
      else if (size cast Pair) {
        preferredSize = (Dimension) size.a;
        minimumSize = (Dimension) size.b;
      }
    }
  };
}