www.entitymodelling.org - entity modelling introduced from first principles - relational database design theory and practice - dependent type theory
When an entity of a particular type may be part of different types of whole, but not simultaneously so, then this is asserted by an arc drawn across the lines representing the composition relationships as here:
This arc is called an exclusion relationship. Note that the presence of the arc circumvents the necessity to dash the relationship lines to represent optionality - optionality of relationships from parts to whole is implied.
This now allows us to improve the model of a file system as shown in figure 8 in which we have introduced a drive entity ( i.e a disk drive) to provide context for those folders which do not themselves have folders for context.