Fix reference_or_constr grammar rule to accept @t as a constr
Fix reference_or_constr grammar rule to accept @t as a constr

With the previous formulation extra parentheses had to be added,
now they are only necessary for compound expressions, like an
application.

git-svn-id: 44dc50bdcc0f5d1a212c497508dc9452e0b35e2b@15954 85f007b7-540e-0410-9357-904b9bb8a0f7

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