code: static JCTree.Tag unoptag(TokenKind token) {
          switch (token) {
          case PLUS:
              return POS;
          case SUB:
              return NEG;
          case BANG:
              return NOT;
          case TILDE:
              return COMPL;
          case PLUSPLUS:
              return PREINC;
          case SUBSUB:
              return PREDEC;
          default:
              return NO_TAG;
          }
      }
docs: Return operation tag of unary operator represented by token,
  No_TAG if token is not a binary operator.