code: 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;
    }
docs: Unpacking keyword arguments will expand a dictionary into a series of keyword arguments.
  
  <p>NOTE: Keyword unpacking behavior is only guaranteed for mapping expressions. Non-mapping
  expressions which attempt to unpack will result in Python runtime errors.
  
  @param mapping The mapping expression to unpack.
  @return This PyFunctionExprBuilder instance.