docs: public PyFunctionExprBuilder setUnpackedKwargs(PyExpr mapping) {
      if (unpackedKwargs != null) {
        throw new UnsupportedOperationException("Only one kwarg unpacking allowed per expression.");
      }
      StringBuilder expr = new StringBuilder("**");
      if (mapping.getPrecedence() < Integer.MAX_VALUE) {
        expr.append("(").append(mapping.getText()).append(")");
      } else {
        expr.append(mapping.getText());
      }
      unpackedKwargs = expr.toString();
      return this;
    }