Problem
Transform functions are how we track changes to eventtypes as events flow through the system. To support the attributes filter, we should implement a transform function to represent how a given eventtype becomes "narrower" as it passes the filter. Specifically, this transform function should (for each attribute set in the attributes filter):

1. Check if the attribute is already set. If it is and does not match the expected value from the filter, return a nil eventtype to model no events passing this filter
2. If the attribute is not already set, set it to the value expected by the filter
3. If the attribute is compatible with the value required by the filter, set it equal to the value required by the filter.

Persona:
Which persona is this feature for?

Exit Criteria
A measurable (binary) test that would indicate that the problem has been resolved.

Time Estimate (optional):
How many developer-days do you think this may take to resolve? 2

Additional context (optional)
Add any other context about the feature request here.