It is a question of semantics. The avoid-runtime-errors-rule would be (in pseudo code): if (isscalar(value)) value; else flatten(value); and it would flatten down scalars and non-nested lists to the same level. Is this desired? Implementing the runtime-errors-rule would be more strict: assert(!isscalar(value)) flatten(value); -- Sent from: http://forum.openscad.org/