Disallow multiple "reply" relationships (Core, clarification)

Section 3 [relationship] states:

  'A reply message MUST contain a [relationship] property consisting 
  of the predefined reply IRI and the message id property of the 
  request message.'

This implies one or more reply relationships must be indicated.  What
does it mean to be a reply to more than one message?

Suggest changing "a" to "exactly one", as in the following rewording:

  'A reply message MUST contain exactly one [relationship] property
  consisting of the predefined reply IRI and the message id property 
  of the request message.'