Schematron Question: proper way to express the text version of assertions?

Hi Folks,

Consider this simple XML document:

<Meeting>
      <StartTime>0900</StartTime>
      <EndTime>1000</EndTime>
</Meeting>

With Schematron I can express this data constraint: 

       "The Meeting's EndTime must be after its StartTime"

This is expressed using a Schematron rule:

<sch:rule context="Meeting">

     <sch:assert test="EndTime &gt; StartTime">
             -- text version of assertion --
     </sch:assert>

</sch:rule>

Read as: "In the context of the Meeting element, I assert that the
value of EndTime must be greater than the value of StartTime"

Question:

Within the <sch:assert> element you can provide a textual description
of the assertion (which I indicate above using "-- text version of
assertion --").  What is the best way of stating the text version of an
assertion: 

         "is", "should be", or "must be"?

For the above example, which of these is best:

(a) EndTime is greater than StartTime

(b) EndTime should be greater than StartTime 

(c) EndTime must be greater than StartTime

Or perhaps something else?

What is recommended?

/Roger