how broker() function in trait SendsPasswordResetEmails return \Illuminate\Contracts\Auth\PasswordBroker?

I am using Laravel 5.3, In Forgot Password Controller, there is Trait SendsPasswordResetEmails

if you go to it's definition, there is function called broker(), it returns contract of type \Illuminate\Contracts\Auth\PasswordBroker

if you go to \Illuminate\Contracts\Auth\PasswordBroker , there is no function declaration with the name of broker()

and not even in it's derived class \Illuminate\Auth\Passwords\PasswordBroker.php

I saw it was present in \Illuminate\Auth\Passwords\PasswordBrokerManager.php

Question: Can you kindly tell how broker() function in trait SendsPasswordResetEmails return \Illuminate\Contracts\Auth\PasswordBroker ?

via Chebli Mohamed