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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def start_ssh_server(port, username, password, namespace):
 """ Start an SSH server on the given port, exposing a Python prompt with the given namespace. """

    # This is a lot of boilerplate, see http://tm.tl/6429 for a ticket to
    # provide a utility function that simplifies this.
    from twisted.internet import reactor
    from twisted.conch.insults import insults
    from twisted.conch import manhole, manhole_ssh
    from twisted.cred.checkers import (
        InMemoryUsernamePasswordDatabaseDontUse as MemoryDB)
    from twisted.cred.portal import Portal

    sshRealm = manhole_ssh.TerminalRealm()
    def chainedProtocolFactory():
        return insults.ServerProtocol(manhole.Manhole, namespace)
    sshRealm.chainedProtocolFactory = chainedProtocolFactory

    sshPortal = Portal(sshRealm, [MemoryDB(**{username: password})])
    reactor.listenTCP(port, manhole_ssh.ConchFactory(sshPortal),
                      interface="127.0.0.1")