This feature does not work:
First here is an invisible block (with {show=0}) defining with a \newcommand definition after this paragraph:
$$
\newcommand{\foo}[0]{bar}
$${show=0}
which renders as:
We make it invisible because this block only contains KaTeX definitions, and should not render to anything.
Then the second math block uses those definitions:
$$
\foo
$$
which renders as:
Analogously with \def, definition:
$$
\gdef\foogdef{bar}
$${show=0}
which renders as:
and the second block using it:
$$
\foogdef
$$
which renders as:
And just to test that {show=1} actually shows, although it is useless, and that {show=0} skips incrementing the equation count:
$$1 + 1$${show=1}
$$2 + 2$${show=0}
$$3 + 3$${show=1}
which renders as:

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