Changes

Refactor post_process of signature merging

The [post_process] function for signature merging was taking two linked
arguments: a [~destructive] flag and a [replace] function to apply only if the
flag was set to [true]. This commit combines the two into a single optional
function [replace] and clarifies some documentation comments.

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Address review comments

- Invert destructive and non-destructive cases
- Move the error inside the computation of the replace function

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