add comment on symbolic test

Author: OriolAbril

tests/xtensor/test_shape.py

@@ -117,6 +117,7 @@ def test_unstack_simple():
 def test_unstack_symbolic():
     x = xtensor(dims=("a", "b", "c"))
     y = stack(x, bc=("b", "c"))
+    # Perform an operation along the stacked index that changes the values but not the length
     y = y[:, ::-1]
     z = unstack(y, bc={"b": x.sizes["b"], "c": x.sizes["c"]})
     x_test = xr_arange_like(xtensor(dims=x.dims, shape=(2, 3, 5)))