diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 992ff8a..d7ed806 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -26,15 +26,32 @@ jobs:
       uses: actions/cache@v1
       with:
         path: .build
-        key: build-${{ github.workspace }}-${{ runner.os }}-${{ matrix.swift-compat-ver }}-${{ hashFiles('Package.*') }}
+        key: build-${{ github.workspace }}-${{ runner.os }}-${{ matrix.swift-compat-ver }}-${{ hashFiles('Package.*') }}-${{ hashFiles('Sources/**/*.swift') }}
         restore-keys: |
+          build-${{ github.workspace }}-${{ runner.os }}-${{ matrix.swift-compat-ver }}-${{ hashFiles('Package.*') }}-
           build-${{ github.workspace }}-${{ runner.os }}-${{ matrix.swift-compat-ver }}-
           build-${{ github.workspace }}-${{ runner.os }}-
           build-${{ github.workspace }}-
     - uses: YOCKOW/Action-setup-swift@master
       with:
-        swift-version: '5.1.4'
+        swift-version: '5.2'
+    - name: Try to build products with debug mode.
+      run: |
+        swift build --configuration debug -Xswiftc -swift-version -Xswiftc ${{ matrix.swift-compat-ver }}
+        if [ $? != 0 ]; then
+          echo "Failed to build products with debug mode."
+          rm -rf $(cd .build/debug && pwd -P)
+        fi
+      continue-on-error: true
     - name: Test with debug mode.
       run: swift test --configuration debug -Xswiftc -swift-version -Xswiftc ${{ matrix.swift-compat-ver }}
+    - name: Try to build products with release mode.
+      run: |
+        swift build --configuration release -Xswiftc -enable-testing -Xswiftc -swift-version -Xswiftc ${{ matrix.swift-compat-ver }}
+        if [ $? != 0 ]; then
+          echo "Failed to build products with release mode."
+          rm -rf $(cd .build/release && pwd -P)
+        fi
+      continue-on-error: true
     - name: Test with release mode.
       run: swift test --configuration release -Xswiftc -enable-testing -Xswiftc -swift-version -Xswiftc ${{ matrix.swift-compat-ver }}
diff --git a/Sources/Ranges/Memoizables.swift b/Sources/Ranges/Memoizables.swift
index 13d91ff..f90ff71 100644
--- a/Sources/Ranges/Memoizables.swift
+++ b/Sources/Ranges/Memoizables.swift
@@ -4,48 +4,54 @@
      Licensed under MIT License.
      See "LICENSE.txt" for more information.
  ************************************************************************************************ */
- 
-/// Immutable multiple ranges.
-/// Results can be memoized.
-public final class MemoizableMultipleRanges<Bound> where Bound: Comparable & Hashable {
-  private var _memoized: [Bound: Bool] = [:]
-  private var _multipleRanges: MultipleRanges<Bound>
-  
-  public init(_ ranges: MultipleRanges<Bound>) {
-    self._multipleRanges = ranges
-  }
-  
-  public func contains(_ value: Bound) -> Bool {
-    func _contains(_ value: Bound) -> Bool {
-      let result = self._multipleRanges.contains(value)
-      self._memoized[value] = result
-      return result
-    }
-    return self._memoized[value] ?? _contains(value)
-  }
-}
-
 
 /// Immutable range-dictionary.
 /// Results can be memoized.
 public final class MemoizableRangeDictionary<Bound, Value> where Bound: Comparable & Hashable {
   private var _memoized: [Bound: Value?] = [:]
+  private var _recentPairs: ArraySlice<RangeDictionary<Bound, Value>._Pair> = []
   private var _rangeDictionary: RangeDictionary<Bound, Value>
   
   public init(_ rangeDictionary: RangeDictionary<Bound, Value>) {
     self._rangeDictionary = rangeDictionary
   }
   
-  public subscript(_ element: Bound) -> Value? {
-    func _value() -> Value? {
-      let result = self._rangeDictionary[element]
-      self._memoized[element] = result
-      return result
+  private func _memoizedValue(for bound: Bound) -> Value?? {
+    return self._memoized[bound]
+  }
+  
+  private func _recentValue(for bound: Bound) -> Value? {
+    for pair in self._recentPairs {
+      if pair.range.contains(bound) { return pair.value }
     }
-    
+    return nil
+  }
+  
+  private func _appendRecentPair(_ pair: RangeDictionary<Bound, Value>._Pair) {
+    self._recentPairs.append(pair)
+    self._recentPairs = self._recentPairs.suffix(3)
+  }
+  
+  private func _valueWithMemoizing(for bound: Bound) -> Value? {
+    if let index = self._rangeDictionary._index(whereRangeContains: bound) {
+      let pair = self._rangeDictionary._rangesAndValues[index]
+      self._memoized[bound] = pair.value
+      self._appendRecentPair(pair)
+      return pair.value
+    } else {
+      self._memoized[bound] = Optional<Value>.none
+      return nil
+    }
+  }
+  
+  public subscript(_ element: Bound) -> Value? {
     switch self._memoized[element] {
     case Optional<Value?>.none:
-      return _value()
+      if let value = self._recentValue(for: element) {
+        self._memoized[element] = value
+        return value
+      }
+      return _valueWithMemoizing(for: element)
     case Optional<Value?>.some(.none):
       return nil
     case Optional<Value?>.some(.some(let value)):
@@ -53,3 +59,21 @@ public final class MemoizableRangeDictionary<Bound, Value> where Bound: Comparab
     }
   }
 }
+
+
+
+/// Immutable multiple ranges.
+/// Results can be memoized.
+public final class MemoizableMultipleRanges<Bound> where Bound: Comparable & Hashable {
+  private var _memoized: MemoizableRangeDictionary<Bound, Void>
+  
+  public init(_ ranges: MultipleRanges<Bound>) {
+    self._memoized = .init(ranges._rangeDictionary)
+  }
+  
+  public func contains(_ value: Bound) -> Bool {
+    return self._memoized[value] != nil
+  }
+}
+
+
diff --git a/Sources/Ranges/MultipleRanges.swift b/Sources/Ranges/MultipleRanges.swift
index 1a2cfb2..32e7fd3 100644
--- a/Sources/Ranges/MultipleRanges.swift
+++ b/Sources/Ranges/MultipleRanges.swift
@@ -17,7 +17,7 @@ private func _bitCastArrayNoneToVoid<T>(_ array: Array<AnyRange<T>>) -> Array<(A
 
 /// Represents multiple ranges.
 public struct MultipleRanges<Bound> where Bound: Comparable {
-  private var _rangeDictionary: RangeDictionary<Bound, Void>
+  internal var _rangeDictionary: RangeDictionary<Bound, Void>
   
   private init(_ rangeDictionary: RangeDictionary<Bound, Void>) {
     self._rangeDictionary = rangeDictionary
diff --git a/Sources/Ranges/RangeDictionary.swift b/Sources/Ranges/RangeDictionary.swift
index bbf3164..d0d6105 100644
--- a/Sources/Ranges/RangeDictionary.swift
+++ b/Sources/Ranges/RangeDictionary.swift
@@ -28,10 +28,10 @@
  
  */
 public struct RangeDictionary<Bound, Value> where Bound: Comparable {
-  fileprivate typealias _Pair = (range: AnyRange<Bound>, value: Value)
+  internal typealias _Pair = (range: AnyRange<Bound>, value: Value)
   
   /// Must be always sorted with the ranges.
-  fileprivate private(set) var _rangesAndValues: [_Pair]
+  internal private(set) var _rangesAndValues: [_Pair]
   private func _twoRanges(from index: Int) -> (AnyRange<Bound>, AnyRange<Bound>) {
     let (pair0, pair1) = self._rangesAndValues._twoElements(from: index)
     return (pair0.range, pair1.range)
@@ -82,7 +82,7 @@ public struct RangeDictionary<Bound, Value> where Bound: Comparable {
     }
   }
   
-  private func _index(whereRangeContains element: Bound) -> Int? {
+  internal func _index(whereRangeContains element: Bound) -> Int? {
     func _binarySearch<C>(_ collection: C, _ element: Bound) -> Int?
       where C: Collection, C.Index == Int, C.Element == _Pair
     {
diff --git a/Tests/RangesTests/MemoizablesTests.swift b/Tests/RangesTests/MemoizablesTests.swift
index 7189644..af09f2c 100644
--- a/Tests/RangesTests/MemoizablesTests.swift
+++ b/Tests/RangesTests/MemoizablesTests.swift
@@ -16,6 +16,7 @@ final class MemoizablesTests: XCTestCase {
     XCTAssertTrue(ranges.contains(555))
     XCTAssertFalse(ranges.contains(5555))
     XCTAssertTrue(ranges.contains(5)) // again
+    XCTAssertTrue(ranges.contains(7)) // near
   }
   
   func test_dictionary() {
@@ -30,5 +31,6 @@ final class MemoizablesTests: XCTestCase {
     XCTAssertEqual(dictionary[25], "20")
     XCTAssertEqual(dictionary[10000], nil)
     XCTAssertEqual(dictionary[5], "0") // again
+    XCTAssertEqual(dictionary[7], "0") // near
   }
 }