In stats::D, I was wondering why variables are represented as symbols
in expressions, but as strings in lists of variables:
D(quote(x^2),"x") => 2*x
D(quote(x^2),quote(x)) => error Variable must be a character string
Strings are not allowed in the expression to denote variables:
D(quote("x"),quote("x")) == D("k","x") => NA (why not an error?)
-s
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