What is the area of a circle?
$${\mathtt{\pi}}{\mathtt{\,\times\,}}{{\mathtt{r}}}^{{\mathtt{2}}} = {\mathtt{3.141\: \!592\: \!653\: \!589\: \!793\: \!2}}{\mathtt{\,\times\,}}{{\left(\underset{{\tiny{\text{Error: Unknown Identifier}}}}{{\mathtt{r}}}\right)}}^{{\mathtt{2}}}$$