diff --git a/Sources/RefdsShared/Foundation/RefdsDictionary.swift b/Sources/RefdsShared/Foundation/RefdsDictionary.swift
index aff176b..d3a702b 100644
--- a/Sources/RefdsShared/Foundation/RefdsDictionary.swift
+++ b/Sources/RefdsShared/Foundation/RefdsDictionary.swift
@@ -7,26 +7,42 @@ public actor RefdsDictionary<
 > {
     private var elements: [Key: Value] = [:]
     private let keyPath: KeyPath<Value, Transform>
+    private let maxCapacity: Int?
     
     public init(
         for type: Value.Type,
-        by keyPath: KeyPath<Value, Transform>
+        by keyPath: KeyPath<Value, Transform>,
+        maxCapacity: Int? = nil
     ) {
         self.keyPath = keyPath
+        self.maxCapacity = maxCapacity
     }
     
     public subscript(key: Key) -> Value? {
         get { elements[key] }
         set {
             guard let newValue else { return remove(for: key) }
+            guard canInsert else { return }
             if elements[key] == nil { insert(for: key) }
             elements[key] = newValue
         }
     }
     
+    @discardableResult
+    public func removeFirst() -> Value? {
+        guard let firstKey = keys.first else { return nil }
+        let element = elements[firstKey]
+        remove(for: firstKey)
+        return element
+    }
+    
+    public var count: Int { keys.count }
     public var keys: [Key] = []
-    public var values: [Value] {
-        keys.compactMap { elements[$0] }
+    public var values: [Value] { keys.compactMap { elements[$0] } }
+    
+    private var canInsert: Bool {
+        guard let maxCapacity else { return true }
+        return count < maxCapacity
     }
     
     private func insert(for key: Key) {