With FILTER_FALSE you remove all false elements from the provided arguments.


FILTER_FALSE(1, 2, false, 3)
This filters out all the false values in the list evaluating to [1, 2, 3].
p is Person
if p.has_email_1 then p.email1 else false end,
if p.has_email_2 then p.email2 else false end,
if p.has_email_3 then p.email3 else false end,
Here, we are only returning a certain provided email of a person if an information has_email is true. If that information is false, we then filter the list with FILTER_FALSE leaving only emails in the list where has_email was provided and true.