Could someone explain the skeletality criterion that Fong and Spivak talk about? They use the term poset to refer to a preorder, and state that a partially ordered set (the usual expansion of the term 'poset') has a requirement of 'skeletality'. From their description, and the one on Wikipedia, I understand that skeletality implies that 'isomorphic objects are necessarily identical'.

Can some one provide examples a _non-skeletal_ preorder? I can't think of one, or I don't understand the concept well.