pasting lemma for smooth functions example

Can someone give me the example of two smooth functions $f(x)$ and $g(x)$ defined on closed set $A$ and $B$ and smooth on that domain such that h(x) defined as follows $$h(x)=\begin{cases}f(x), & \text{if x $\in$ A}\\ g(x), & \text{ if x $\in$ B} \end{cases}$$is smooth on $A \cup B$? same as pasting lemma.