@Nullable
  private Object evaluatePrefix(@NotNull PyPrefixExpression expression) {
    if (expression.getOperator() == PyTokenTypes.NOT_KEYWORD) {
      final Boolean value = PyUtil.as(evaluate(expression.getOperand()), Boolean.class);
      if (value != null) {
        return !value;
      }
    }
    else if (expression.getOperator() == PyTokenTypes.MINUS) {
      final Number number = PyUtil.as(evaluate(expression.getOperand()), Number.class);
      if (number != null) {
        return fromBigInteger(toBigInteger(number).negate());
      }
    }

    return null;
  }
@Nullable
  private Object evaluatePrefix(@NotNull PyPrefixExpression expression)