read isn't written yet, but I think all the building blocks I need for it are.

Compiles and runs; does nothing yet.

src/c/payloads/cons.c

@@ -14,7 +14,11 @@
 #include "memory/pso.h"
 #include "memory/pso2.h"
 #include "memory/tags.h" 
+
 #include "payloads/cons.h"
+#include "payloads/exception.h"
+
+#include "ops/string_ops.h"
 
 /**
  * @brief allocate a cons cell with this car and this cdr, and return a pointer
@@ -58,19 +62,29 @@ struct pso_pointer car( struct pso_pointer cons ) {
 }
 
 /**
- * @brief return the cdr of this cons cell.
+ * @brief return the cdr of this cons (or other sequence) 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 cdr( struct pso_pointer p ) {
     struct pso_pointer result = nil;
     struct pso2 *object = pointer_to_object( result );
 
-    if ( consp( cons ) ) {
-        result = object->payload.cons.cdr;
+    switch (get_tag_value( p)) {
+    case CONSTV : result = object->payload.cons.cdr; break;
+    case KEYTV :
+    case STRINGTV :
+    case SYMBOLTV :
+    	result = object->payload.string.cdr; break;
+    default :
+    	result = make_exception(
+    			cons(c_string_to_lisp_string(L"Invalid type for cdr"), p),
+				nil, nil);
+    	break;
     }
+
     // TODO: else throw an exception
 
     return result;

src/c/payloads/exception.h

@@ -24,6 +24,7 @@ struct exception_payload {
     struct pso_pointer cause;
 };
 
-struct pso_pointer make_exception( struct pso_pointer message, struct pso_pointer frame_pointer, struct pso_pointer cause);
+struct pso_pointer make_exception( struct pso_pointer message,
+		struct pso_pointer frame_pointer, struct pso_pointer cause);
 
 #endif

src/c/payloads/stack.h

@@ -37,4 +37,6 @@ struct stack_frame_payload {
     uint32_t depth;
 };
 
+struct pso_pointer make_frame( struct pso_pointer previous, ...);
+
 #endif