The post-edit hook section claimed a `%let` "creates a new variable in the
hook's own scope and is silently discarded" while `call symputx` updates the
existing one. That is not how SAS resolves it: LIBREF and DS already exist in
an enclosing symbol table (mpe_loader's local table), and a macro only gets a
new local when the name is absent from every enclosing table, so both forms
assign to the same variable. Reworded to state that, and to keep the practical
advice (call symputx is explicit and does not depend on the enclosing scope
still holding the name).
Raised in review of #16.
dc
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