In ConTeXt, I want to lift a \hat slightly up, since \hat is too low and touches the variable.
Now, this post is exactly what I want.
It suggests:
\hat{\rule{0ex}{1.2ex}\mkern-3mu x}_{t+1}
However, ConTeXt seemingly doesn't recognize \rule.
I don't understand the nature of \rule, and I can't find explanation in The TEXbook nor in TEX for the Impatient.
Is \rule a LaTeX function?
Where can I find the function definition, and how can I implement it using (perhaps) plain TEX and ConTeXt functions?
Or are there other ways to achieve the result?

