[exim] canonical local_part from an address

By chance, I noticed that an arbitrary string representing an address
(for example, items in $recipients) contains something like
"iamquoted"@???, then the ${local_part:} string operator
preserves the quotes. Obviously, exim knows how to get rid of the
quotes when it does lookups and such, but I couldn't find an easy way to
get a canonical local_part myself using available string operators. (I
know I could using matching and such to peel off the leading and
trailing double quotes, but there are lots more RFC-822, et seq, quoting
possibilities.)

Have I overlooked it or is there no built-in operator for this?