diff --git a/package.json b/package.json
index 955f0b5..29d1d15 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@samchon/openapi",
-  "version": "0.1.2",
+  "version": "0.1.4",
   "description": "",
   "main": "./lib/index.js",
   "typings": "./lib/index.d.ts",
diff --git a/src/OpenApi.ts b/src/OpenApi.ts
index 3e4ff51..5708fb1 100644
--- a/src/OpenApi.ts
+++ b/src/OpenApi.ts
@@ -8,6 +8,33 @@ import { SwaggerV2Converter } from "./internal/SwaggerV2Converter";
 /**
  * Emended OpenAPI v3.1 definition used by `typia` and `nestia`.
  *
+ * `OpenApi` is a namespace containing functions and interfaces for emended
+ * OpenAPI v3.1 specification. The keyword "emended" means that `OpenApi` is
+ * not a direct OpenAPI v3.1 specification ({@link OpenApiV3_1}), but a little
+ * bit shrinked to remove ambiguous and duplicated expressions of OpenAPI v3.1
+ * for the convenience of `typia` and `nestia`.
+ *
+ * For example, when representing nullable type, OpenAPI v3.1 supports three ways.
+ * In that case, `OpenApi` remains only the third way, so that makes `typia` and
+ * `nestia` (especially `@nestia/editor`) to be simple and easy to implement.
+ *
+ * 1. `type: ["string", "null"]`
+ * 2. `type: "string", nullable: true`
+ * 3. `oneOf: [{ type: "string" }, { type: "null" }]`
+ *
+ * Here is the entire list of differences between OpenAPI v3.1 and emended `OpenApi`.
+ *
+ * - Operation
+ *   - Merged {@link OpenApiV3_1.IPathItem.parameters} to {@link OpenApi.IOperation.parameters}
+ *   - Resolved {@link OpenApi.IJsonSchema.IReference references} of {@link OpenApiV3_1.IOperation} mebers
+ * - JSON Schema
+ *   - Decomposed mixed type: {@link OpenApiV3_1.IJsonSchema.IMixed}
+ *   - Resolved nullable property: {@link OpenApiV3_1.IJsonSchema.__ISignificant.nullable}
+ *   - Array type utilizes only single {@link OpenAPI.IJsonSchema.IArray.items}
+ *   - Tuple type utilizes only {@link OpenApi.IJsonSchema.ITuple.prefixItems}
+ *   - Merged {@link OpenApiV3_1.IJsonSchema.IAnyOf} to {@link OpenApi.IJsonSchema.IOneOf}
+ *   - Merged {@link OpenApiV3_1.IJsonSchema.IRecursiveReference} to {@link OpenApi.IJsonSchema.IReference}
+ *
  * @author Jeongho Nam - https://github.com/samchon
  */
 export namespace OpenApi {
@@ -152,6 +179,7 @@ export namespace OpenApi {
     | IJsonSchema.INumber
     | IJsonSchema.IString
     | IJsonSchema.IArray
+    | IJsonSchema.ITuple
     | IJsonSchema.IObject
     | IJsonSchema.IReference
     | IJsonSchema.IOneOf
@@ -220,7 +248,6 @@ export namespace OpenApi {
       /** @type uint */ maxItems?: number;
     }
     export interface ITuple extends __ISignificant<"array"> {
-      items: never;
       prefixItems: IJsonSchema[];
       additionalItems: boolean | IJsonSchema;
       /** @type uint */ minItems?: number;
diff --git a/src/OpenApiV3_1.ts b/src/OpenApiV3_1.ts
index c9ba01d..47396b5 100644
--- a/src/OpenApiV3_1.ts
+++ b/src/OpenApiV3_1.ts
@@ -278,9 +278,13 @@ export namespace OpenApiV3_1 {
     export interface IReference<Key = string> extends __IAttribute {
       $ref: Key;
     }
+    export interface IRecursiveReference extends __IAttribute {
+      $recursiveRef: string;
+    }
 
     export interface __ISignificant<Type extends string> extends __IAttribute {
       type: Type;
+      nullable?: boolean;
     }
     export interface __IAttribute {
       title?: string;
diff --git a/src/internal/OpenApiV3Converter.ts b/src/internal/OpenApiV3Converter.ts
index 418ba43..5d8b751 100644
--- a/src/internal/OpenApiV3Converter.ts
+++ b/src/internal/OpenApiV3Converter.ts
@@ -204,6 +204,7 @@ export namespace OpenApiV3Converter {
       ),
     };
     const visit = (schema: OpenApiV3.IJsonSchema): void => {
+      // NULLABLE PROPERTY
       if (
         (schema as OpenApiV3.IJsonSchema.__ISignificant<any>).nullable === true
       )
diff --git a/src/internal/OpenApiV3_1Converter.ts b/src/internal/OpenApiV3_1Converter.ts
index b19f812..d209626 100644
--- a/src/internal/OpenApiV3_1Converter.ts
+++ b/src/internal/OpenApiV3_1Converter.ts
@@ -224,7 +224,15 @@ export namespace OpenApiV3_1Converter {
         ),
       ),
     };
+    const nullable: { value: boolean } = { value: false };
+
     const visit = (schema: OpenApiV3_1.IJsonSchema): void => {
+      // NULLABLE PROPERTY
+      if (
+        (schema as OpenApiV3_1.IJsonSchema.__ISignificant<any>).nullable ===
+        true
+      )
+        nullable.value ||= true;
       // MIXED TYPE CASE
       if (TypeChecker.isMixed(schema)) {
         if (schema.const !== undefined)
@@ -407,7 +415,7 @@ export namespace OpenApiV3_1Converter {
           });
       }
       // OBJECT TYPE CASE
-      else if (TypeChecker.isObject(schema)) {
+      else if (TypeChecker.isObject(schema))
         union.push({
           ...schema,
           ...{
@@ -428,12 +436,24 @@ export namespace OpenApiV3_1Converter {
               : undefined,
           },
         });
-      }
+      else if (TypeChecker.isRecursiveReference(schema))
+        union.push({
+          ...schema,
+          ...{
+            $ref: schema.$recursiveRef,
+            $recursiveRef: undefined,
+          },
+        });
       // THE OTHERS
       else union.push(schema);
     };
 
     visit(input);
+    if (
+      nullable.value === true &&
+      !union.some((e) => (e as OpenApi.IJsonSchema.INull).type === "null")
+    )
+      union.push({ type: "null" });
     return {
       ...(union.length === 0
         ? { type: undefined }
@@ -477,6 +497,11 @@ export namespace OpenApiV3_1Converter {
       schema: OpenApiV3_1.IJsonSchema,
     ): schema is OpenApiV3_1.IJsonSchema.IReference =>
       (schema as OpenApiV3_1.IJsonSchema.IReference).$ref !== undefined;
+    export const isRecursiveReference = (
+      schema: OpenApiV3_1.IJsonSchema,
+    ): schema is OpenApiV3_1.IJsonSchema.IRecursiveReference =>
+      (schema as OpenApiV3_1.IJsonSchema.IRecursiveReference).$recursiveRef !==
+      undefined;
     export const isOneOf = (
       schema: OpenApiV3_1.IJsonSchema,
     ): schema is OpenApiV3_1.IJsonSchema.IOneOf =>