Let $\theta=\{x_1\leftarrow t_1,\dots,x_n\leftarrow t_n\},\sigma =\{y_1\leftarrow s_1,\dots,y_m\leftarrow s_m\}$ and $\lambda=\{z_1\leftarrow r_1,\dots,z_k\leftarrow r_k\}$ be substitutions.
Prove that composition of substitutions it is not commutative.
So we want to see that in general $\theta\circ\sigma\ne\sigma\circ\theta.$
What could be the counterexample so that we can have $\theta\circ\sigma\ne\sigma\circ\theta$ ?
How can I construct the counterexample?
Help me please?