ios - how to add an extension to Observable and generics

Observable+Extensions.swift

import Foundation
import RxSwift
import RxCocoa

extension ObservableType where E == String {
    
    /// Limit the max size of the string that's been emitted
    func limit(maxStringSize: Int) -> RxSwift.Observable<String> {
        return scan("") { (previous, new) -> String in
            return new.count <= maxStringSize ? new : previous
        }
    }
    
}