I am using context minimals, with the simpleslides module to
make a presentation.

I noticed that when typesetting math formulas, I get missing characters.

I finally tracked it down to an issue (stupidity from me?) with the
"context" executable.

I made a new file, with some example math markup, posted below
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\starttext

And this is some maths with PostScript fonts.
\startformula
{a \over b} = {c \over d} \iff ad - bc = 0,
\qquad \int_0^{2\pi}\!\!\! \sin^2(x)dx =\pi,
\qquad {\rm e}^{{\rm i}\pi} + 1 = 0,
\qquad \sum_{k=1}^{\infty}{1 \over k^2} = {\pi^2 \over 6}.
\stopformula
Some ligatures: ffi, ffl.

\stoptext
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