resource sem4()
    op sema() {send}
    op semb() {send}

    process a  {
	write(1111);
	send semb()
	receive sema()
	write(2222)
	send semb()
    }

    process b  {
	receive semb()
	write(3333)
	send sema()
	receive semb()
	write(4444)
    }

end