diff --git a/README.md b/README.md
index 20e7ef1..02ecdcd 100644
--- a/README.md
+++ b/README.md
@@ -175,6 +175,8 @@ Support levels:
 <li><code>init(_: String)</code></li>
 <li><code>var isNewline: Bool</code></li>
 <li><code>var isWhitespace: Bool</code></li>
+<li><code>var isUppercase: Bool</code></li>
+<li><code>var isLowercase: Bool</code></li>
 <li><code>func lowercased() -> String</code></li>
 <li><code>func uppercased() -> String</code></li>
           </ul>
diff --git a/Sources/SkipLib/Skip/Collections.kt b/Sources/SkipLib/Skip/Collections.kt
index d5df107..f61d9ee 100644
--- a/Sources/SkipLib/Skip/Collections.kt
+++ b/Sources/SkipLib/Skip/Collections.kt
@@ -775,6 +775,8 @@ val IntRange.upperBound: Int
     get() = if (endInclusive == Int.MAX_VALUE) Int.MAX_VALUE else endInclusive + 1
 val IntRange.lowerBound: Int
     get() = start
+val IntRange.isEmpty: Boolean
+    get() = isEmpty()
 
 // IntRange.isEmpty, IntRange.contains can be used as-is
 
diff --git a/Sources/SkipLib/Skip/String.kt b/Sources/SkipLib/Skip/String.kt
index f16f851..3a44824 100644
--- a/Sources/SkipLib/Skip/String.kt
+++ b/Sources/SkipLib/Skip/String.kt
@@ -39,8 +39,8 @@ fun String(repeating: String, count: Int): String {
 // Swift.String API
 fun String.lowercased(): String = lowercase()
 fun String.uppercased(): String = uppercase()
-fun Substring.lowercased(): Substring = Substring(stringValue.lowercased(), startIndex)
-fun Substring.uppercased(): Substring = Substring(stringValue.uppercase(), startIndex)
+fun Substring.lowercased(): String = stringValue.lowercased()
+fun Substring.uppercased(): String = stringValue.uppercase()
 
 fun String.hasPrefix(prefix: String): Boolean = startsWith(prefix)
 fun String.hasPrefix(prefix: Substring): Boolean = startsWith(prefix.stringValue)
@@ -58,7 +58,7 @@ fun Substring.contains(string: String): Boolean = stringValue.contains(string)
 fun Substring.contains(string: Substring): Boolean = stringValue.contains(string)
 
 // String.plus(Any?) used as-is
-operator fun Substring.plus(string: Any): Substring = Substring(string.toString(), startIndex)
+operator fun Substring.plus(string: Any): String = stringValue + string
 
 // MARK: - Sequence
 
@@ -511,6 +511,10 @@ val Char.isNewline: Boolean
         else -> false
     }
 
+val Char.isUppercase: Boolean
+    get() = isUpperCase()
+val Char.isLowercase: Boolean
+    get() = isLowerCase()
 fun Char.uppercased(): String = toString().uppercased()
 fun Char.lowercased(): String = toString().lowercased()
 
diff --git a/Sources/SkipLib/String.swift b/Sources/SkipLib/String.swift
index d641dac..3341ad4 100644
--- a/Sources/SkipLib/String.swift
+++ b/Sources/SkipLib/String.swift
@@ -189,12 +189,10 @@ public struct Character {
         fatalError()
     }
 
-    @available(*, unavailable)
     public var isUppercase: Bool {
         fatalError()
     }
 
-    @available(*, unavailable)
     public var isLowercase: Bool {
         fatalError()
     }