User blog comment:Hyp cos/R function II/@comment-5529393-20150406080531/@comment-5529393-20150406111045

Another question: take {0 B 0} where B is some brace expression. We should scan to the second 0 and rule A2 should apply - except the expression isn't {0} like the rule says. But perhaps you mean it is equivalent to {0}?

Also, I imagine there should be a rule disallowing a separator on the outside, since then we can never scan inside the separator.