Two properties or none

Hi all.

I need to model a given constraint:
in a given set of resources, two properties must appear together, or none.

Let me be more specific:
any resource with a property "ex:a" of type "xsd:string" must be tested
against the <IssueShape> shape
and must PASS.

At the moment, here are my shapes:
###############
PREFIX ex: <http://ex.example/#>
PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>

start = <IssueShape>

<IssueShape> {
    ex:a   xsd:string,
    (ex:state xsd:string,
     ex:state2 xsd:string)?
}
###################

and here is my set of data:
###################
PREFIX ex: <http://ex.example/#>
PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>

<Issue1>
ex:a "a"
.
<Issue2>
  ex:state        "a";
  ex:a "a"
.
#################

Here are my expectations:
<Issue2> is supposed to fail, whereas <Issue1> is supposed to be ok.

My experience with the online demos are:

-----
1) with http://www.w3.org/2013/ShEx/SimpleShExDemo
-----

<Issue1> PASS.
<Issue2> is not tested.
<Issue2> is in the remaining triples:
  <Issue2> <http://ex.example/#state> "a".
  <Issue2> <http://ex.example/#a> "a".

-----
2) with http://www.w3.org/2013/ShEx/FancyShExDemo and Pre-typed graph node:
<Issue1>
-----

PASS

-----
3) with http://www.w3.org/2013/ShEx/FancyShExDemo and Pre-typed graph node:
<Issue2>
-----

FAIL {
Errors:
 ☹   mixed matches of     <http://ex.example/#state> <
http://www.w3.org/2001/XMLSchema#string>,
    <http://ex.example/#state2> <http://www.w3.org/2001/XMLSchema#string>
passed: [[        PASS {
      <http://ex.example/#state> <http://www.w3.org/2001/XMLSchema#string>
matched by <Issue2> <http://ex.example/#state> "a".
    }]]
empty: [[        FAIL {
      <http://ex.example/#state2> <http://www.w3.org/2001/XMLSchema#string>
permitted to not match
    }]]
Matches:
  <http://ex.example/#a> <http://www.w3.org/2001/XMLSchema#string> matched
by <Issue2> <http://ex.example/#a> "a".
  <http://ex.example/#state> <http://www.w3.org/2001/XMLSchema#string>
matched by <Issue2> <http://ex.example/#state> "a".
  <http://ex.example/#state2> <http://www.w3.org/2001/XMLSchema#string>
permitted to not match
}

-----
4) with http://www.w3.org/2013/ShEx/FancyShExDemo and selecting "Find node
types"
-----

I get this message: <Issue1> is a <IssueShape>
And no list of remaining triples.

----------------------------------------------------------------------------------------------------------------------------------

In short, I never get a report that clearly states:
<Issue1> was tested against the <IssueShape> shape and PASSed.
<Issue2> was tested against the <IssueShape> shape and FAILed.


Is there a way to get such a report with the actual ShEx spec and
implementation?
Is my requirement related to the concept of closed shapes? (still under
discussion, afaiu).