vote on issue #152 Add schematron rules for productionset reference constraints

Issue
152, Add schematron rules for productionset
reference constraints.  Norm says  In a productionset there
are a couple of referential integrity constraints that should be
checked with Schematron, <productionrecap> and
<constraint>.
https://github.com/docbook/docbook/issues/152
I think this is a useful addition, and I don't see any negative
consequences.  If there are no objections, I move to accept this
change.  Scott can second it and vote, and other voting TC members
can vote.  I vote yes.

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Bob Stayton