coroutine cor1 {        coroutine cor2 {
  BEGIN
    ...
    RESUME cor2;--------->BEGIN
                            ...
    ...<--------------------RESUME cor1;
    ...
    RESUME cor2;----------->...
                            ...
    ...<--------------------RESUME cor1;
    ...
    RESUME cor2;----------->...
                            ...
    ...<--------------------RESUME cor1;
    ...                   END;
  END;                 }
}