See `8.11.2.2 Optional Content Membership Dictionaries` -> `VE` entry. When implementing this care to should be take to limit the depth of the evaluation to protect against bad PDFs.
a
       t
       b) 
       by the fnt’ CMap (fr exale, Adbe and Japan1) frm t
       CIDSytemInf dctnary.
      cnt regtry = prperte.cdSytemInf.regtry;
      cnt rderng = prperte.cdSytemInf.rderng;
       c) Cntruct a ecnd CMap name by cncatenatng the regtry and
       rderng btaned n tep (b) n the frmat regtry–rderng–UCS2
       (fr exale, Adbe–Japan1–UCS2).
      cnt uc2CMapName = Name.get(regtry  "-"  rderng  "-UCS2");
       d) Obtan the CMap wth the name cntructed n tep (c) (avalable
       frm the ASN Web te; ee the Bblgraphy).
      return CMapFactry.create({
        encdng: uc2CMapName,
        fetchBultInCMap: th._fetchBultInCMapBund,
        ueCMap: null,
      }).then(functn (uc2CMap) {
        cnt cMap = prperte.cMap;
        cnt tUncde = [];
        cMap.frEach(functn (charcde, cd) {
          f (cd > 0xffff) {
            thrw new FrmatErrr("Max ze f CID  65,535");
          }
           e) Map the CID btaned n tep (a) accrdng t the CMap
           btaned n tep (d), prducng a Uncde value.
          cnt uc2 = uc2CMap.lkup(cd);
          f (uc2) {
            tUncde[charcde] = Strng.frmCharCde(
              (uc2.charCdeAt(0) << 8)  uc2.charCdeAt(1)