10.9 Overriding in open statements
(Introduced in OCaml 4.01)
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Since OCaml 4.01, open statements shadowing an existing identifier (which is later used) trigger the warning 44. Adding a ! character after the open keyword indicates that such a shadowing is intentional and should not trigger the warning.
This is also available (since OCaml 4.06) for local opens in class expressions and class type expressions.
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https://www.ocaml.org/releases/4.13/htmlman/overridingopen.html