Okay, looks like I got bit by the "Reading Is Fundamental" bug...
The short answer is CALCULATETABLE.
When I actually OPENED Ferrari & Russo's DAX Patterns book, there's a chapter on Surveys (Chapter 13)... the question in the book is "Which customers gave the same answers for both Question1 and Question2?" and the answer is:
CALCULATE (
COUNTROWS ( Customers ),
CALCULATETABLE (
Answers,
USERELATIONSHIP (Answers[AnswerKey], Filter2[AnswerKey] )
),
CALCULATETABLE (
Answers,
USERELATIONSHIP (Anwers[AnswerKey], Filter1[AnswerKey] )
)
)
In case someone is looking for this, it's here[/url].
Yep, "Here's your sign."
... after more playing around with this... I got this working... I guess CALCULATETABLE is something I need to figure out...
-- has HF
CALCULATETABLE('Patient',FILTER('Diagnoses',[SymptomName]="Heart Failure"))
-- has D
CALCULATETABLE('Patient',FILTER('Diagnoses',[SymptomName]="Diabetes"))
-- has both:
INTERSECT('has D', 'has HF')
or
INTERSECT (
CALCULATETABLE('Patient',FILTER('Diagnoses',[SymptomName]="Diabetes")),
CALCULATETABLE('Patient',FILTER('Diagnoses',[SymptomName]="Heart Failure"))
)