locked main form conditionally validate 1 subform based on other subform

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I have a main form that has 3 fields. They are all locked fields, only
to show the user the "person" record that they are looking at. They
update to subforms on a tab. The first subform is activities (like
from a mailing - received return no forwarding address, received
updated address, called us, etc)

The other subform is for addresses, original and updated.

There is a date field for the date of any updated addresses.

What I need to do is allow users to enter activities and move around
from main record to main record. The important thing is that if they
enter an activity on the subform that is for receiving an address
change, I need to make sure that they do not leave the main record,
without entering in an updated address for that same date, on the
address subform.

The activity subform has a date field too, so the validation part seems
like it wouldnt be too hard...

IIF ?subform-activities-field-activitytype = "updated address" (3/code
to lookup table) AND ?subform-address-field-date = grrr this is where I
get confused.

Then msg "Stop, you must add the updated address for the activity of
receiving an address change"
cancel = true

end if

make sense?

Any help would be gratefully appreciated!