private Node buildFieldDecl(FieldDecl fd) throws SemanticException {
    Type t = fd.declType();

    if (t.isSubtype(SJ_PROTOCOL_TYPE)) {
      if (!(fd instanceof SJFieldProtocolDecl)) {
        throw new SemanticException(
            "[SJProtocolDeclTypeBuilder] Protocols may only be declared using the protocol keyword: "
                + fd);
      }

      SJTypeNode tn = disambiguateSJTypeNode(this, ((SJProtocolDecl) fd).sessionType());
      SJSessionType st = tn.type();
      String sjname = fd.name(); // Should match that given by SJVariable.sjname.

      SJFieldInstance fi = (SJFieldInstance) fd.fieldInstance();
      SJFieldProtocolInstance fpi =
          sjts.SJFieldProtocolInstance((SJFieldInstance) fd.fieldInstance(), st, sjname);

      fpi.setConstantValue(
          fi
              .constantValue()); // Currently, constant checker not run on custom nodes/type
                                 // objects. (Previously done by SJNoAliasTypeBuilder.)

      fd = fd.fieldInstance(fpi);
      fd = (FieldDecl) setSJProtocolDeclExt((SJProtocolDecl) fd, tn, sjname);

      updateSJFieldInstance(fi, fpi);
    }

    return fd;
  }