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rspec.rb:12 requires rspec/core/formatters/progress_formatter. This in turn requires rspec/core/formatters/base_text_formatter, which has a line which depends on the standard set module, without actually requiring it.
As a result, an attempt to require ci/reporter/rspec into a context which has not already required set will fail.
This recently broke some code written by a colleague of mine (and debugged by me).
Ideally, base_text_formatter would itself require set. If the RSpec maintainers recognise this as a bug and fix it, then this problem will go away. However, if they don't, and instead assert that having set required is a prerequisite for the use of base_text_formatter, then i would humbly submit that this constitutes a bug in CI::Reporter, the fix for which would be to add this require to rspec.rb.
An RSpec dude was happy for their code to change, so i changed it. So this bug is revealed to have been an illusion all along.