def delete_features(host_name, client_name, client_pass, feature_names=None):
    """
    Remove a number of numerical features in the client. If a list  is not provided, remove all features.

    Inputs: - host_name: A string containing the address of the machine where the PServer instance is hosted.
            - client_name: The PServer client name.
            - client_pass: The PServer client's password.
            - feature_names: A python list of feature names.
    """
    # Get all features.
    if feature_names is None:
        feature_names = get_feature_names(host_name,
                                          client_name,
                                          client_pass)

    # Remove all features.
    feature_to_be_removed = ("&".join(["ftr=%s"]*len(feature_names))) % tuple(feature_names)
    features_req = construct_request("pers",
                                     client_name,
                                     client_pass,
                                     'remftr',
                                     feature_to_be_removed)
    send_request(host_name,
                 features_req)