Update multivariate.py

Author: bwalters13

pymc/distributions/multivariate.py

@@ -410,12 +410,7 @@ def dist(cls, alpha, *, Sigma=None, mu=0, scale=None, tau=None, chol=None, lower
             scale = Sigma
         alpha = pt.as_tensor_variable(floatX(alpha))
         mu = pt.as_tensor_variable(floatX(mu))
-        aCa = pt.matmul(pt.matmul(alpha, scale), alpha)
-        delta = pt.matmul((1/ pt.sqrt(1 + aCa)) * scale, alpha)
-        a = pt.concatenate((pt.ones(1), delta)).reshape((1, -1))
-        b = pt.concatenate((delta[:, None], scale), axis=1)
-        cov_star = pt.concatenate((a, b))
-        scale = cov_star
+        scale = quaddist_matrix(scale, chol, tau, lower)
         # PyTensor is stricter about the shape of mu, than PyMC used to be
         mu, _ = pt.broadcast_arrays(mu, scale[..., -1])