const c = circuit(5);
// 1. encode logical |+_L>
c.h(0);
c.cx(0, 1);
c.cx(0, 2);
// 2. inject bit-flip on the middle data qubit
c.x(1);
// 3. syndrome extraction: Z0Z1 -> ancilla 3, Z1Z2 -> ancilla 4
c.cx(0, 3);
c.cx(1, 3);
c.cx(1, 4);
c.cx(2, 4);
// 4. correction implied by syndrome (1,1): flip qubit 1 back
c.x(1);
return c;