set contents of aRef to contents of aRef as text -- caution: AS silently ignores invalid TID values, so separator items must be explicitly validated to catch any user errors; for now, just coerce to text and catch errors, but might want to make it more rigorous in future (e.g. if a list of lists is given, should sublist be treated as an error instead of just coercing it to text, which is itself TIDs sensitive); see also existing TODO on TypeSupport's asTextParameter handler