byte req[4];
int t=0;
int p=0;
byte v=0;


active proctype cabin() { 

idle: if
:: v>0; goto mov; 

fi;
mov: if
:: t==p; goto open; 

::  d_step {t<p;p = p-1;}  goto mov; 

::  d_step {t>p;p = p+1;}  goto mov; 

fi;
open: if
::  d_step {req[p] = 0;v = 0;}  goto idle; 

fi;
}

active proctype environment() { 

read: if
::  d_step {req[0]==0;req[0] = 1;}  goto read; 

::  d_step {req[1]==0;req[1] = 1;}  goto read; 

::  d_step {req[2]==0;req[2] = 1;}  goto read; 

::  d_step {req[3]==0;req[3] = 1;}  goto read; 

fi;
}

active proctype controller() { 
byte ldir=0;

wait: if
::  d_step {v==0;t = t+(2*ldir)-1;}  goto work; 

fi;
work: if
::  d_step {t<0 || t==4;ldir = 1-ldir;}  goto wait; 

:: t>=0 && t<4 && req[t]==1; goto done; 

::  d_step {t>=0 && t<4 && req[t]==0;t = t+(2*ldir)-1;}  goto work; 

fi;
done: if
:: v = 1; goto wait; 

fi;
}