diff --git a/src/rpc/blockchain.cpp b/src/rpc/blockchain.cpp
--- a/src/rpc/blockchain.cpp
+++ b/src/rpc/blockchain.cpp
@@ -1172,7 +1172,6 @@
         LOCK(g_mempool.cs);
         CCoinsViewMemPool view(pcoinsTip.get(), g_mempool);
         if (!view.GetCoin(out, coin) || g_mempool.isSpent(out)) {
-            // TODO: this should be done by the CCoinsViewMemPool
             return NullUniValue;
         }
     } else {
diff --git a/src/txmempool.h b/src/txmempool.h
--- a/src/txmempool.h
+++ b/src/txmempool.h
@@ -770,6 +770,13 @@
 /**
  * CCoinsView that brings transactions from a memorypool into view.
  * It does not check for spendings by memory pool transactions.
+ * Instead, it provides access to all Coins which are either unspent in the
+ * base CCoinsView, or are outputs from any mempool transaction!
+ * This allows transaction replacement to work as expected, as you want to
+ * have all inputs "available" to check signatures, and any cycles in the
+ * dependency graph are checked directly in AcceptToMemoryPool.
+ * It also allows you to sign a double-spend directly in signrawtransaction,
+ * as long as the conflicting transaction is not yet confirmed.
  */
 class CCoinsViewMemPool : public CCoinsViewBacked {
 protected: