Well, we have a REPL. It blows up horribly, but we have one.

src/c/payloads/write_stream.c

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+/**
+ *  payloads/read_stream.c
+ *
+ *  A read stream.
+ *
+ *  (c) 2026 Simon Brooke <simon@journeyman.cc>
+ *  Licensed under GPL version 2.0, or, at your option, any later version.
+ */
+
+#include <stdio.h>
+
+#include <curl/curl.h>
+
+#include "io/fopen.h"
+#include "memory/pointer.h"
+#include "memory/pso.h"
+#include "memory/pso2.h"
+#include "memory/tags.h"
+
+
+/**
+ * Construct a cell which points to a stream open for writing.
+ * @param input the C stream to wrap.
+ * @param metadata a pointer to an associaton containing metadata on the stream.
+ * @return a pointer to the new read stream.
+ */
+struct pso_pointer make_write_stream( URL_FILE *output,
+                                      struct pso_pointer metadata ) {
+    struct pso_pointer pointer = allocate( WRITETAG, 2 );
+    struct pso2 *cell = pointer_to_object( pointer );
+
+    cell->payload.stream.stream = output;
+    cell->payload.stream.meta = metadata;
+
+    return pointer;
+}