Pipe outputPipe = new Pipe(runtime);
httpReq.setInput("source", inputPipe);
httpReq.setOutput("result", outputPipe);
TreeWriter req = new TreeWriter(runtime);
req.startDocument(step.getNode().getBaseURI());
req.addStartElement(XProcConstants.c_request);
req.addAttribute(_method, "HEAD");
req.addAttribute(_href, uri.toASCIIString());
req.addAttribute(_status_only, "true");
req.addAttribute(_detailed, "true");