I would prefer that operators be matched hygienically and not by name.
(I apologize for not mentioning this until the last call, too many
things were happening and this slipped my mind...)

Then the implementation could use identifier properties for extending
the syntax. There has been more support for identifier properties in
Scheme implementation since you emailed WG2 about it: Capy Scheme[1] now
supports them. Also, in some implementations one can use other
mechanisms besides properties: see SRFI 262's implementation section[2].

Also, in the absence of define-property, the core `expr` syntax can be
defined in terms of syntax-rules.

Regarding the "import of any library" issue, the (srfi 266) library can
re-export all relevant identifiers. There may be an issue if one wants
to define "fx>>" and hence overwrites the fx>> in the SRFI. However,
using the import renaming system, one can get around that. (Also, if one
has identifier properties, then the property could be added to the
user's fx>>, getting around this problem).

[1]: https://codeberg.org/playXE/capy
[2]: https://srfi.schemers.org/srfi-262/srfi-262.html#implementation

-- Peter McGoron