interface IPort {
    void open(); 
    // connect port to file

    int read(); 
    // read one integer from file, if the file isn't exhausted
    // @pre: !out_of_intsP()

    boolean out_of_intsP(); 
    // is the file content exhausted? 

    void close();     
    // disconnect the port from the file 

    // sequence contract: open x {read | out_of_intsP}* x close
}