Huge amount of work. Does not even nearly compile, but it's nearer.

src/c/payloads/cons.c

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+/**
+ *  payloads/cons.h
+ *
+ *  A cons cell.
+ *
+ *  (c) 2026 Simon Brooke <simon@journeyman.cc>
+ *  Licensed under GPL version 2.0, or, at your option, any later version.
+ */
+
+#include <stdbool.h>
+
+#include "memory/node.h"
+#include "memory/pointer.h"
+#include "memory/pso.h"
+#include "payloads/cons.h"
+
+/**
+ * @brief allocate a cons cell with this car and this cdr, and return a pointer
+ * to it.
+ * 
+ * @param car the pointer which should form the car of this cons cell;
+ * @param cdr the pointer which should form the cdr of this cons cell.
+ * @return struct pso_pointer a pointer to the newly allocated cons cell.
+ */
+struct pso_pointer cons( struct pso_pointer car, struct pso_pointer cdr) {
+    struct pso_pointer result = allocate( CONSTAG, 2);
+
+    struct pso2 *object = pointer_to_object( result );
+    object->payload.cons.car = car;
+    object->payload.cons.cdr = cdr;
+
+    inc_ref( car);
+    inc_ref( cdr);
+
+    return result;
+}
+
+/**
+ * @brief return true if `ptr` indicates a cons cell, else false.
+ * 
+ * @param ptr a pointer.
+ * @return true if `ptr` indicates a cons cell.
+ * @return false otherwise
+ */
+bool consp( struct pso_pointer ptr) {
+    // TODO: make it actually work!
+    return false;
+}
+
+/**
+ * @brief return the car of this cons cell.
+ * 
+ * @param cons a pointer to the cell.
+ * @return the car of the indicated cell.
+ * @exception if the pointer does not indicate a cons cell.
+ */
+struct pso_pointer car( struct pso_pointer cons) {
+    struct pso_pointer result = nil;
+    struct pso2 *object = pointer_to_object( result );
+
+    if ( consp( cons)) {
+        result = object->payload.cons.car;
+    }
+    // TODO: else throw an exception
+
+    return result;
+}
+
+/**
+ * @brief return the cdr of this cons cell.
+ * 
+ * @param cons a pointer to the cell.
+ * @return the cdr of the indicated cell.
+ * @exception if the pointer does not indicate a cons cell.
+ */
+struct pso_pointer cdr( struct pso_pointer cons) {
+    struct pso_pointer result = nil;
+    struct pso2 *object = pointer_to_object( result );
+
+    if ( consp( cons)) {
+        result = object->payload.cons.cdr;
+    }
+    // TODO: else throw an exception
+
+    return result;
+}