error: uncertain reuse without correlation annotation
  --> line 2:20
   |
 2 | let variance_est = a * a;
   |                    ^^^^^ `a` appears twice in this product
   |
   = note: treating repeated occurrences of `a` as independent understates
     the true variance of the result.
   = help: use `square(a)` for the correct self-product variance formula,
     or wrap with `correlated(..., ..., cov = ...)` if you have an explicit
     covariance estimate.
