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Sorry, sent prematurely...

The example I was starting to write was:

function a() <_ENV,...>

is almost equivalent to an unannotated function except that it forces
the `_ENV` upvalue to exist, and forces it to be in the first

The value now is that a function declared as:

function a() <...>

Can take whatever upvalues it wants from the locals in its hierarchy
of closures but can never access the implied `_ENV` variable/upvalue.

There can be an argument made that disabling the implied lookup from
the `_ENV` parameter is different to disabling access to the `_ENV`
parameter entirely, but given that any variable or argument with the
name `_ENV` is automatically blessed as the source of globals I don't
think there is any problem similarly treating it as special when
disabling it as an upvalue.