The paper proves by assertional means the correctness of a construction of an atomic shared variable for one writer and one reader. This construction uses four unsafe variables and four safe boolean variables. Assignment to a safe but nonatomic variable is modelled as a repetition of random assignments concluded by an actual assignment. The proof obligation consists of four invariants. These are proved using 25 auxiliary invariants. The proof has been constructed and verified with the theorem prover NQTHM.