code: def sum_layout_dimensions(dimensions):
      
      min = sum([d.min for d in dimensions if d.min is not None])
      max = sum([d.max for d in dimensions if d.max is not None])
      preferred = sum([d.preferred for d in dimensions])
  
      return LayoutDimension(min=min, max=max, preferred=preferred)
docs: Sum a list of :class:`.LayoutDimension` instances.