Magritte choice of fields

Is there a clever way to specify a choice of fields in magritte? I mean a magritte container consists of a number of child descriptions. Each of them describes the constraints for itself. But when it comes to define constraints for a subset of them it gets awkward pretty quick. What I want is a condition that I can use to check if one field _or_ the other is specified. Or more general if any or all of a subset of the children match. I can add for every case a condition on the container to check for a particular case but programmatically it is much harder. The condition of the individual description needs to have something between optional and mandatory. Before I try to work something out I wanted to know if there is any kind of solution already present somewhere. thanks, norbert