semantics of daml:UnambiguousProperty

A little question re DAML+OIL semantics. If a property is an unambigous
property, does the equality specified by DAML+OIL below hold only at a
particular point in time or does it hold across time and change?

Eg. if I say some resource X has a foo:contact property
whose value is some other resource Y, where Y is "the resource  whose
personalMailbox is mailto:lighthouse-keeper@stonyisland.example.com", what
does this mean? Might Y differ over time, so long as at any one time there
was only a single resource with that property/value pair. What inferences
does daml:UnambiguousProperty license in this respect?

Dan


http://www.daml.org/2000/12/daml+oil.daml

<rdfs:Class rdf:ID="UnambiguousProperty">
  <rdfs:label xml:lang="en">UnambiguousProperty</rdfs:label>
  <rdfs:comment>
    if P is an UnambiguousProperty, then if P(x, y) and P(z, y) then x=z.
    aka injective. e.g. if nameOfMonth(m, "Feb")
    and nameOfMonth(n, "Feb") then m and n are the same month.
  </rdfs:comment>
  <rdfs:subClassOf
rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Property"/>
</rdfs:Class>


http://www.daml.org/2000/12/reference.html

       an UnambigousProperty element.
       This asserts that an instance y can only be the value of P for a
single instance x, i.e: there cannot be two distinct instances x1 and x2
such that both (x1,y) and
       (x2,y) are both instances of P.