Implementing NSCopying, NSMutableCopying for immutable class with mutable counterpart

Node.h

#import <Foundation/Foundation.h>

@interface Node : NSObject <NSCopying, NSMutableCopying>
@property (nonatomic, weak, readonly) Node *parent;
@property (nonatomic, strong, readonly) Node *left;
@property (nonatomic, strong, readonly) Node *right;
- (instancetype)initWithParent:(Node *)parent left:(Node *)left right:(Node *)right;
@end

@interface MutableNode : Node
@property (nonatomic, weak) Node *parent;
@property (nonatomic, strong) Node *left;
@property (nonatomic, strong) Node *right;
@end

Node.m

#import "Node.h"

@interface Node ()
@property (nonatomic, weak) Node *parent;
@property (nonatomic, strong) Node *left;
@property (nonatomic, strong) Node *right;
@end

@implementation Node
- (instancetype)initWithParent:(Node *)parent left:(Node *)left right:(Node *)right {
    self = [super init];
    if (!self) {
        return nil;
    }
    self.parent = parent;
    self.left = left;
    self.right = right;
    return self;
}
- (id)copyWithZone:(NSZone *)zone {
    return self;
}
- (id)mutableCopyWithZone:(NSZone *)zone {
    MutableNode *node = [[MutableNode allocWithZone:zone] init];
    node.parent = self.parent;
    node.left = self.left;
    node.right = self.right;
    return node;
}
- (void)setLeft:(Node *)left {
    _left = left;
    if (left.parent != self) {
        left.parent = self;
    }
}
- (void)setRight:(Node *)right {
    _right = right;
    if (right.parent != self) {
        right.parent = self;
    }
}
- (NSString *)description {
    NSMutableString *string = [NSMutableString stringWithFormat:@"<%@:%p>", [self class], self];
    if (!self.parent && !self.left && !self.right) {
        return [string copy];
    }
    [string appendString:@" {"];
    if (self.parent) {
        [string appendFormat:@" parent = <%@:%p>", [self.parent class], self.parent];
    }
    if (self.left) {
        [string appendFormat:@" left = <%@:%p>", [self.left class], self.left];
    }
    if (self.right) {
        [string appendFormat:@" right = <%@:%p>", [self.right class], self.right];
    }
    [string appendString:@" }"];
    return [string copy];
}
@end

@implementation MutableNode
- (id)copyWithZone:(NSZone *)zone {
    Node *node = [[Node allocWithZone:zone] init];
    [node setValue:self.parent forKey:@"parent"];
    [node setValue:self.left forKey:@"left"];
    [node setValue:self.right forKey:@"right"];
    return node;
}
@end