An infix separator currently lays down this parser:

<alt>
<seq>
<ArrayPosGreaterThan />
<Sep>, ddd List(, ddd, ; ddd, ddd)</Sep>
</seq>
<seq>
<GroupPosGreaterThan />
<Sep>, ddd List(, ddd, ; ddd, ddd)</Sep>
</seq>
<Nada />
</alt>

This should be replaced by a primitive InfixSepPrim which prints out as

<InfixSep>, ddd List(, ddd, ; ddd, ddd)</InfixSep>

And which implements the same logic but within the primitive. There's no
performance advantage (and some disadvantage) to elaborating the infix sep logic
in the grammar.