ensure links only belong to 1 list

also some small documentation changes.

src/nickel/linkedlist.c

@@ -2,6 +2,7 @@
 #include <stdint.h>
 #include <stdbool.h>
 #include <stdio.h>
+#include <assert.h>
 
 #include "nickel/linkedlist.h"
 
@@ -19,6 +20,10 @@ bool ni_list__is_empty(ni_list* l) {
 }
 
 void ni_list__insert_after(ni_list* l, ni_list_node* entry, ni_list_node* n) {
+    // n must not be in a list already, 
+    // if you need an object in multiple lists, use multiple links.
+    assert(n->next == NULL && n->prev == NULL);
+
     // setup our incomming node as the next node after the insert point
     n->next = entry->next;
     n->prev = entry;
@@ -32,6 +37,10 @@ void ni_list__insert_after(ni_list* l, ni_list_node* entry, ni_list_node* n) {
 }
 
 void ni_list__insert_before(ni_list* l, ni_list_node* entry, ni_list_node* n) {
+    // n must not be in a list already, 
+    // if you need an object in multiple lists, use multiple links.
+    assert(n->next == NULL && n->prev == NULL);
+    
     // setup our incomming node as the next node before the insert point
     n->prev = entry->prev;
     n->next = entry;