I discovered (serendipitously) that the mods\physicalDescription\extent can be repeated and the mods record still validates.  For the collection I'm working with, repeating this field seems to make sense.  A list of the components that make up the object seems more readable than a lengthy concatenated string:

                        <mods:form authority="marcform">electronic</mods:form>
                        <mods:extent>8 black-and-white negatives (60mm)</mods:extent>
                        <mods:extent>9 color negatives (35mm)</mods:extent>
                        <mods:digitalOrigin>reformatted digital</mods:digitalOrigin>

Any reactions to this?  Is repeating the field risky because of the possibility that a future version of the .xsd may constrain number
mods\physicalDescription\extent to a single instance?