  A conforming triangular mixed element recently proposed by Hu and Zhang for
linear elasticity is extended by rearranging the global degrees of freedom.
More precisely, adaptive meshes $\mathcal{T}_1$, $\cdots$, $\mathcal{T}_N$
which are successively refined from an initial mesh $\mathcal{T}_0$ through a
newest vertex bisection strategy, admit a crucial hierarchical structure,
namely, a newly added vertex $\boldsymbol{x}$ of the mesh $\mathcal{T}_\ell$ is
the midpoint of an edge $e$ of the coarse mesh $\mathcal{T}_{\ell-1}$. Such a
hierarchical structure is explored to partially relax the $C^0$ vertex
continuity of symmetric matrix-valued functions in the discrete stress space of
the original element on $\mathcal{T}_\ell$ and results in an extended discrete
stress space. A feature of this extended discrete stress space is its
nestedness in the sense that a space on a coarse mesh $\mathcal{T}$ is a
subspace of a space on any refinement $\hat{\mathcal{T}}$ of $\mathcal{T}$,
which allows a proof of convergence of a standard adaptive algorithm. The idea
is extended to impose a general traction boundary condition on the discrete
level. Numerical experiments are provided to illustrate performance on both
uniform and adaptive meshes.
