code: private OperatorSymbol enterUnop(String name,
                                       Type arg,
                                       Type res,
                                       int opcode) {
          OperatorSymbol sym =
              new OperatorSymbol(makeOperatorName(name),
                                 new MethodType(List.of(arg),
                                                res,
                                                List.<Type>nil(),
                                                methodClass),
                                 opcode,
                                 predefClass);
          predefClass.members().enter(sym);
          return sym;
      }
docs: Enter a unary operation into symbol table.
  @param name     The name of the operator.
  @param arg      The type of the operand.
  @param res      The operation's result type.
  @param opcode   The operation's bytecode instruction.