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.
hadw,
  trngTPDFStrng,
  TextRenderngMde,
  UNSUPPORTED_FEATURES,
  Utl,
  warn,
} frm "../hared/utl.j";
rt { CMapFactry, IdenttyCMap } frm "./cmap.j";
rt {
  Cmd,
  Dct,
  EOF,
  Dct,
  Name,
  Ref,
  Stream,
  Name,
  Ref,
  RefSet,
} frm "./prmtve.j";
rt { DecdeStream, NullStream } frm "./tream.j";
rt {
  ErrrFnt,
  Fnt,
  FntFlag,
  getFntType,
  IdenttyTUncdeMap,
  TUncdeMap,
} frm "./fnt.j";
rt {