diff --git a/src/OpenApiTypeChecker.ts b/src/OpenApiTypeChecker.ts
new file mode 100644
index 0000000..1d1d4ff
--- /dev/null
+++ b/src/OpenApiTypeChecker.ts
@@ -0,0 +1,348 @@
+import { OpenApi } from "./OpenApi";
+import { MapUtil } from "./utils/MapUtil";
+
+export namespace OpenApiTypeChecker {
+  export const visit =
+    (closure: (schema: OpenApi.IJsonSchema) => void) =>
+    (components: OpenApi.IComponents) => {
+      const already: Set<string> = new Set();
+      const next = (schema: OpenApi.IJsonSchema): void => {
+        closure(schema);
+        if (OpenApiTypeChecker.isReference(schema)) {
+          const key: string = schema.$ref.split("#/components/schemas/").pop()!;
+          if (already.has(key) === true) return;
+          already.add(key);
+          const found: OpenApi.IJsonSchema | undefined =
+            components.schemas?.[key];
+          if (found !== undefined) next(found);
+        } else if (OpenApiTypeChecker.isOneOf(schema))
+          schema.oneOf.forEach(next);
+        else if (OpenApiTypeChecker.isObject(schema)) {
+          for (const value of Object.values(schema.properties ?? {}))
+            next(value);
+          if (
+            typeof schema.additionalProperties === "object" &&
+            schema.additionalProperties !== null
+          )
+            next(schema.additionalProperties);
+        } else if (OpenApiTypeChecker.isArray(schema)) next(schema.items);
+        else if (OpenApiTypeChecker.isTuple(schema)) {
+          (schema.prefixItems ?? []).forEach(next);
+          if (
+            typeof schema.additionalItems === "object" &&
+            schema.additionalItems !== null
+          )
+            next(schema.additionalItems);
+        }
+      };
+      return next;
+    };
+
+  /* -----------------------------------------------------------
+    TYPE CHECKERS
+  ----------------------------------------------------------- */
+  export const isNull = (
+    schema: OpenApi.IJsonSchema,
+  ): schema is OpenApi.IJsonSchema.INull =>
+    (schema as OpenApi.IJsonSchema.INull).type === "null";
+  export const isUnknown = (
+    schema: OpenApi.IJsonSchema,
+  ): schema is OpenApi.IJsonSchema.IUnknown =>
+    (schema as OpenApi.IJsonSchema.IUnknown).type === undefined &&
+    !isConstant(schema) &&
+    !isOneOf(schema) &&
+    !isReference(schema);
+
+  export const isConstant = (
+    schema: OpenApi.IJsonSchema,
+  ): schema is OpenApi.IJsonSchema.IConstant =>
+    (schema as OpenApi.IJsonSchema.IConstant).const !== undefined;
+  export const isBoolean = (
+    schema: OpenApi.IJsonSchema,
+  ): schema is OpenApi.IJsonSchema.IBoolean =>
+    (schema as OpenApi.IJsonSchema.IBoolean).type === "boolean";
+  export const isInteger = (
+    schema: OpenApi.IJsonSchema,
+  ): schema is OpenApi.IJsonSchema.IInteger =>
+    (schema as OpenApi.IJsonSchema.IInteger).type === "integer";
+  export const isNumber = (
+    schema: OpenApi.IJsonSchema,
+  ): schema is OpenApi.IJsonSchema.INumber =>
+    (schema as OpenApi.IJsonSchema.INumber).type === "number";
+  export const isString = (
+    schema: OpenApi.IJsonSchema,
+  ): schema is OpenApi.IJsonSchema.IString =>
+    (schema as OpenApi.IJsonSchema.IString).type === "string";
+
+  export const isArray = (
+    schema: OpenApi.IJsonSchema,
+  ): schema is OpenApi.IJsonSchema.IArray =>
+    (schema as OpenApi.IJsonSchema.IArray).type === "array" &&
+    (schema as OpenApi.IJsonSchema.IArray).items !== undefined;
+  export const isTuple = (
+    schema: OpenApi.IJsonSchema,
+  ): schema is OpenApi.IJsonSchema.ITuple =>
+    (schema as OpenApi.IJsonSchema.ITuple).type === "array" &&
+    (schema as OpenApi.IJsonSchema.ITuple).prefixItems !== undefined;
+  export const isObject = (
+    schema: OpenApi.IJsonSchema,
+  ): schema is OpenApi.IJsonSchema.IObject =>
+    (schema as OpenApi.IJsonSchema.IObject).type === "object";
+  export const isReference = (
+    schema: OpenApi.IJsonSchema,
+  ): schema is OpenApi.IJsonSchema.IReference =>
+    (schema as any).$ref !== undefined;
+  export const isOneOf = (
+    schema: OpenApi.IJsonSchema,
+  ): schema is OpenApi.IJsonSchema.IOneOf =>
+    (schema as OpenApi.IJsonSchema.IOneOf).oneOf !== undefined;
+
+  /* -----------------------------------------------------------
+    OPERATORS
+  ----------------------------------------------------------- */
+  export const covers = (
+    components: OpenApi.IComponents,
+  ): ((x: OpenApi.IJsonSchema, y: OpenApi.IJsonSchema) => boolean) =>
+    coverStation(components)(new Map());
+
+  const coverStation =
+    (components: OpenApi.IComponents) =>
+    (visited: Map<OpenApi.IJsonSchema, Map<OpenApi.IJsonSchema, boolean>>) =>
+    (x: OpenApi.IJsonSchema, y: OpenApi.IJsonSchema): boolean => {
+      const cache: boolean | undefined = visited.get(x)?.get(y);
+      if (cache !== undefined) return cache;
+
+      // FOR RECURSIVE CASE
+      const nested: Map<OpenApi.IJsonSchema, boolean> = MapUtil.take(visited)(
+        x,
+      )(() => new Map());
+      nested.set(y, true);
+
+      // COMPUTE IT
+      const result: boolean = coverSchema(components)(visited)(x, y);
+      nested.set(y, result);
+      return result;
+    };
+
+  const coverSchema =
+    (components: OpenApi.IComponents) =>
+    (visited: Map<OpenApi.IJsonSchema, Map<OpenApi.IJsonSchema, boolean>>) =>
+    (x: OpenApi.IJsonSchema, y: OpenApi.IJsonSchema): boolean => {
+      // CHECK EQUALITY
+      if (x === y) return true;
+      else if (isReference(x) && isReference(y) && x.$ref === y.$ref)
+        return true;
+
+      // COMPARE WITH FLATTENING
+      const alpha: OpenApi.IJsonSchema[] = flatSchema(components)(x);
+      const beta: OpenApi.IJsonSchema[] = flatSchema(components)(y);
+      if (alpha.some((x) => isUnknown(x))) return true;
+      else if (beta.some((x) => isUnknown(x))) return false;
+      return beta.every((b) =>
+        alpha.some((a) => coverEscapedSchema(components)(visited)(a, b)),
+      );
+    };
+
+  const coverEscapedSchema =
+    (components: OpenApi.IComponents) =>
+    (visited: Map<OpenApi.IJsonSchema, Map<OpenApi.IJsonSchema, boolean>>) =>
+    (x: OpenApi.IJsonSchema, y: OpenApi.IJsonSchema): boolean => {
+      // CHECK EQUALITY
+      if (x === y) return true;
+      else if (isUnknown(x)) return true;
+      else if (isUnknown(y)) return false;
+      else if (isNull(x)) return isNull(y);
+      // ATOMIC CASE
+      else if (isConstant(x)) return isConstant(y) && x.const === y.const;
+      else if (isBoolean(x))
+        return isBoolean(y) || (isConstant(y) && typeof y.const === "boolean");
+      else if (isInteger(x))
+        return (isInteger(y) || isConstant(y)) && coverInteger(x, y);
+      else if (isNumber(x))
+        return (
+          (isConstant(y) || isInteger(y) || isNumber(y)) && coverNumber(x, y)
+        );
+      else if (isString(x))
+        return (isConstant(y) || isString(y)) && coverString(x, y);
+      // INSTANCE CASE
+      else if (isArray(x))
+        return (
+          (isArray(y) || isTuple(y)) && coverArray(components)(visited)(x, y)
+        );
+      else if (isObject(x))
+        return isObject(y) && coverObject(components)(visited)(x, y);
+      else if (isReference(x)) return isReference(y) && x.$ref === y.$ref;
+      return false;
+    };
+
+  const coverArray =
+    (components: OpenApi.IComponents) =>
+    (visited: Map<OpenApi.IJsonSchema, Map<OpenApi.IJsonSchema, boolean>>) =>
+    (
+      x: OpenApi.IJsonSchema.IArray,
+      y: OpenApi.IJsonSchema.IArray | OpenApi.IJsonSchema.ITuple,
+    ): boolean => {
+      if (isTuple(y))
+        return (
+          y.prefixItems.every((v) =>
+            coverStation(components)(visited)(x.items, v),
+          ) &&
+          (y.additionalItems === undefined ||
+            (typeof y.additionalItems === "object" &&
+              coverStation(components)(visited)(x.items, y.additionalItems)))
+        );
+      else if (
+        !(
+          x.minItems === undefined ||
+          (y.minItems !== undefined && x.minItems <= y.minItems)
+        )
+      )
+        return false;
+      else if (
+        !(
+          x.maxItems === undefined ||
+          (y.maxItems !== undefined && x.maxItems >= y.maxItems)
+        )
+      )
+        return false;
+      return coverStation(components)(visited)(x.items, y.items);
+    };
+
+  const coverObject =
+    (components: OpenApi.IComponents) =>
+    (visited: Map<OpenApi.IJsonSchema, Map<OpenApi.IJsonSchema, boolean>>) =>
+    (
+      x: OpenApi.IJsonSchema.IObject,
+      y: OpenApi.IJsonSchema.IObject,
+    ): boolean => {
+      if (!x.additionalProperties && !!y.additionalProperties) return false;
+      else if (
+        !!x.additionalProperties &&
+        !!y.additionalProperties &&
+        ((typeof x.additionalProperties === "object" &&
+          y.additionalProperties === true) ||
+          (typeof x.additionalProperties === "object" &&
+            typeof y.additionalProperties === "object" &&
+            !coverStation(components)(visited)(
+              x.additionalProperties,
+              y.additionalProperties,
+            )))
+      )
+        return false;
+      return Object.entries(y.properties ?? {}).every(([key, b]) => {
+        const a: OpenApi.IJsonSchema | undefined = x.properties?.[key];
+        if (a === undefined) return false;
+        else if (
+          (x.required?.includes(key) ?? false) === true &&
+          (y.required?.includes(key) ?? false) === false
+        )
+          return false;
+        return coverStation(components)(visited)(a, b);
+      });
+    };
+
+  const coverInteger = (
+    x: OpenApi.IJsonSchema.IInteger,
+    y: OpenApi.IJsonSchema.IConstant | OpenApi.IJsonSchema.IInteger,
+  ): boolean => {
+    if (isConstant(y))
+      return typeof y.const === "number" && Number.isInteger(y.const);
+    return [
+      x.type === y.type,
+      x.minimum === undefined ||
+        (y.minimum !== undefined && x.minimum <= y.minimum),
+      x.maximum === undefined ||
+        (y.maximum !== undefined && x.maximum >= y.maximum),
+      x.exclusiveMinimum !== true ||
+        x.minimum === undefined ||
+        (y.minimum !== undefined &&
+          (y.exclusiveMinimum === true || x.minimum < y.minimum)),
+      x.exclusiveMaximum !== true ||
+        x.maximum === undefined ||
+        (y.maximum !== undefined &&
+          (y.exclusiveMaximum === true || x.maximum > y.maximum)),
+      x.multipleOf === undefined ||
+        (y.multipleOf !== undefined &&
+          y.multipleOf / x.multipleOf ===
+            Math.floor(y.multipleOf / x.multipleOf)),
+    ].every((v) => v);
+  };
+
+  const coverNumber = (
+    x: OpenApi.IJsonSchema.INumber,
+    y:
+      | OpenApi.IJsonSchema.IConstant
+      | OpenApi.IJsonSchema.IInteger
+      | OpenApi.IJsonSchema.INumber,
+  ): boolean => {
+    if (isConstant(y)) return typeof y.const === "number";
+    return [
+      x.type === y.type || (x.type === "number" && y.type === "integer"),
+      x.minimum === undefined ||
+        (y.minimum !== undefined && x.minimum <= y.minimum),
+      x.maximum === undefined ||
+        (y.maximum !== undefined && x.maximum >= y.maximum),
+      x.exclusiveMinimum !== true ||
+        x.minimum === undefined ||
+        (y.minimum !== undefined &&
+          (y.exclusiveMinimum === true || x.minimum < y.minimum)),
+      x.exclusiveMaximum !== true ||
+        x.maximum === undefined ||
+        (y.maximum !== undefined &&
+          (y.exclusiveMaximum === true || x.maximum > y.maximum)),
+      x.multipleOf === undefined ||
+        (y.multipleOf !== undefined &&
+          y.multipleOf / x.multipleOf ===
+            Math.floor(y.multipleOf / x.multipleOf)),
+    ].every((v) => v);
+  };
+
+  const coverString = (
+    x: OpenApi.IJsonSchema.IString,
+    y: OpenApi.IJsonSchema.IConstant | OpenApi.IJsonSchema.IString,
+  ): boolean => {
+    if (isConstant(y)) return typeof y.const === "string";
+    return [
+      x.format === undefined ||
+        (y.format !== undefined && coverFormat(x.format, y.format)),
+      x.pattern === undefined || x.pattern === y.pattern,
+      x.minLength === undefined ||
+        (y.minLength !== undefined && x.minLength <= y.minLength),
+      x.maxLength === undefined ||
+        (y.maxLength !== undefined && x.maxLength >= y.maxLength),
+    ].every((v) => v);
+  };
+
+  const coverFormat = (
+    x: Required<OpenApi.IJsonSchema.IString>["format"],
+    y: Required<OpenApi.IJsonSchema.IString>["format"],
+  ): boolean =>
+    x === y ||
+    (x === "idn-email" && y === "email") ||
+    (x === "idn-hostname" && y === "hostname") ||
+    (["uri", "iri"].includes(x) && y === "url") ||
+    (x === "iri" && y === "uri") ||
+    (x === "iri-reference" && y === "uri-reference");
+
+  const flatSchema =
+    (components: OpenApi.IComponents) =>
+    (schema: OpenApi.IJsonSchema): OpenApi.IJsonSchema[] => {
+      schema = escapeReference(components)(schema);
+      if (isOneOf(schema))
+        return schema.oneOf.map(flatSchema(components)).flat();
+      return [schema];
+    };
+
+  const escapeReference =
+    (components: OpenApi.IComponents) =>
+    (
+      schema: OpenApi.IJsonSchema,
+    ): Exclude<OpenApi.IJsonSchema, OpenApi.IJsonSchema.IReference> =>
+      isReference(schema)
+        ? escapeReference(components)(
+            components.schemas![
+              schema.$ref.replace("#/components/schemas/", "")
+            ],
+          )
+        : schema;
+}
diff --git a/src/index.ts b/src/index.ts
index f569352..1681d00 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,6 +1,7 @@
 export * from "./IMigrateRoute";
 export * from "./IMigrateDocument";
 export * from "./OpenApi";
+export * from "./OpenApiTypeChecker";
 
 export * from "./SwaggerV2";
 export * from "./OpenApiV3";
diff --git a/src/internal/MigrateRouteConverter.ts b/src/internal/MigrateRouteConverter.ts
index 9fddabd..0500b16 100644
--- a/src/internal/MigrateRouteConverter.ts
+++ b/src/internal/MigrateRouteConverter.ts
@@ -1,8 +1,8 @@
 import { IMigrateRoute } from "../IMigrateRoute";
 import { OpenApi } from "../OpenApi";
+import { OpenApiTypeChecker } from "../OpenApiTypeChecker";
 import { Escaper } from "../utils/Escaper";
 import { StringUtil } from "../utils/StringUtil";
-import { OpenApiTypeChecker } from "./OpenApiTypeChecker";
 
 export namespace MigrateRouteConverter {
   export interface IProps {
diff --git a/src/internal/OpenApiTypeChecker.ts b/src/internal/OpenApiTypeChecker.ts
index 6d84f8f..64f9a56 100644
--- a/src/internal/OpenApiTypeChecker.ts
+++ b/src/internal/OpenApiTypeChecker.ts
@@ -1,59 +1,2 @@
-import { OpenApi } from "../OpenApi";
-
-export namespace OpenApiTypeChecker {
-  export const isNull = (
-    schema: OpenApi.IJsonSchema,
-  ): schema is OpenApi.IJsonSchema.INull =>
-    (schema as OpenApi.IJsonSchema.INull).type === "null";
-  export const isUnknown = (
-    schema: OpenApi.IJsonSchema,
-  ): schema is OpenApi.IJsonSchema.IUnknown =>
-    (schema as OpenApi.IJsonSchema.IUnknown).type === undefined &&
-    !isConstant(schema) &&
-    !isOneOf(schema) &&
-    !isReference(schema);
-
-  export const isConstant = (
-    schema: OpenApi.IJsonSchema,
-  ): schema is OpenApi.IJsonSchema.IConstant =>
-    (schema as OpenApi.IJsonSchema.IConstant).const !== undefined;
-  export const isBoolean = (
-    schema: OpenApi.IJsonSchema,
-  ): schema is OpenApi.IJsonSchema.IBoolean =>
-    (schema as OpenApi.IJsonSchema.IBoolean).type === "boolean";
-  export const isInteger = (
-    schema: OpenApi.IJsonSchema,
-  ): schema is OpenApi.IJsonSchema.IInteger =>
-    (schema as OpenApi.IJsonSchema.IInteger).type === "integer";
-  export const isNumber = (
-    schema: OpenApi.IJsonSchema,
-  ): schema is OpenApi.IJsonSchema.INumber =>
-    (schema as OpenApi.IJsonSchema.INumber).type === "number";
-  export const isString = (
-    schema: OpenApi.IJsonSchema,
-  ): schema is OpenApi.IJsonSchema.IString =>
-    (schema as OpenApi.IJsonSchema.IString).type === "string";
-
-  export const isArray = (
-    schema: OpenApi.IJsonSchema,
-  ): schema is OpenApi.IJsonSchema.IArray =>
-    (schema as OpenApi.IJsonSchema.IArray).type === "array" &&
-    (schema as OpenApi.IJsonSchema.IArray).items !== undefined;
-  export const isTuple = (
-    schema: OpenApi.IJsonSchema,
-  ): schema is OpenApi.IJsonSchema.ITuple =>
-    (schema as OpenApi.IJsonSchema.ITuple).type === "array" &&
-    (schema as OpenApi.IJsonSchema.ITuple).prefixItems !== undefined;
-  export const isObject = (
-    schema: OpenApi.IJsonSchema,
-  ): schema is OpenApi.IJsonSchema.IObject =>
-    (schema as OpenApi.IJsonSchema.IObject).type === "object";
-  export const isReference = (
-    schema: OpenApi.IJsonSchema,
-  ): schema is OpenApi.IJsonSchema.IReference =>
-    (schema as any).$ref !== undefined;
-  export const isOneOf = (
-    schema: OpenApi.IJsonSchema,
-  ): schema is OpenApi.IJsonSchema.IOneOf =>
-    (schema as OpenApi.IJsonSchema.IOneOf).oneOf !== undefined;
-}
+// FOR LEGACY VERSIONS
+export * from "../OpenApiTypeChecker";
diff --git a/src/internal/OpenApiV3Downgrader.ts b/src/internal/OpenApiV3Downgrader.ts
index c273207..f16acf1 100644
--- a/src/internal/OpenApiV3Downgrader.ts
+++ b/src/internal/OpenApiV3Downgrader.ts
@@ -1,6 +1,6 @@
 import { OpenApi } from "../OpenApi";
+import { OpenApiTypeChecker } from "../OpenApiTypeChecker";
 import { OpenApiV3 } from "../OpenApiV3";
-import { OpenApiTypeChecker } from "./OpenApiTypeChecker";
 
 export namespace OpenApiV3Downgrader {
   export interface IComponentsCollection {
diff --git a/src/internal/SwaggerV2Downgrader.ts b/src/internal/SwaggerV2Downgrader.ts
index cb7f389..aa4bdb0 100644
--- a/src/internal/SwaggerV2Downgrader.ts
+++ b/src/internal/SwaggerV2Downgrader.ts
@@ -1,6 +1,6 @@
 import { OpenApi } from "../OpenApi";
+import { OpenApiTypeChecker } from "../OpenApiTypeChecker";
 import { SwaggerV2 } from "../SwaggerV2";
-import { OpenApiTypeChecker } from "./OpenApiTypeChecker";
 
 export namespace SwaggerV2Downgrader {
   export interface IComponentsCollection {