WARNING: this is just a draft and may be changed!
A XSubject represents a grouping of related information for a single entity,
such as a person.
Such information includes the subject's identities as well as its
security-related attributes (passwords and cryptographic keys, for example).
Subjects may potentially have multiple identities.
Each identity is represented as a principal within the subject.
Principals simply bind names to a subject.
For example, a subject that happens to be a person, Alice, might have two
principals: one which binds "Alice Bar", the name on her driver license,
to the subject, and another which binds, "999-99-9999", the number on her
student identification card, to the subject.
Both principals refer to the same subject even though each has a different
name.