Is explicit substitution supposed to work with a function's `with` environment? Because it's not.

[eleses]

The manual says (emphasis mine)

Explicit substitution can be used to customize a component or any expression with a lexical environment by replacing some of its internal definitions, without having to modify it.

But:

prs2(x,y) = p, p + q with { p = x; q = y; };

process = prs2[p = 10; q = 20;];

And the outputs are zero, so this not a SVG bug.