%%

Bturn;
Aturn;

x;
!x;

y;
!y;

a1;  -;     x;     a2;
a2;  -;     Bturn; a3;
a3;  !y;    -;     acs;
a3;  Aturn; -;     acs;
acs; -;     !x;    a5.

b1;  -;     y;     b2;
b2;  -;     Aturn; b3;
b3;  !x;    -;     bcs;
b3;  Bturn; -;     bcs;
bcs; -;     !y;    b5.

ok. acs,bcs; -; _error.