How to get textbook display of an expression held in a string?

[BruceH]

I have a piece of code that generates an expression as a string. I can't find a way to convert it into a CAS expression (within the program and without simplifying or evaluating it) so that it displays pretty-printed in textbook mode on the history stack when the program ends.

Typical example is: "3 NTHROOT (1/8)" which I'd like to see displayed as:
\begin{equation}
\sqrt[3]{1/8}
\end{equation}

Any suggestions, please?