Count Distinct Possible Overlap

[ThePeterK]

I am trying to count the number of students who registered for classes in a term who also registered for classes in the sequentially-previous term on the chart. The term ID numbers may have gaps in the sequence. 

Example

Scenario 1: I have no filters.

In Term X-0, I see the current number of students who also registered in Term X-1,
In Term X-1, I see the number of students who were registered in Term X-1 that were also registered in Term X-2.
In Term X-2, I see the number of students who were registered in Term X-2 that were also registered in Term X-3
..and so on to Term X-X, the first term, where there were no students registered in a prior term, empty

Scenario 2: I have Terms filtered on Term 0, Term X-1 and Term X-3

In Term X-0, I see the current number of students who also registered in Term X-1
In Term X-1, I see the number of students who were registered in Term X-1 that were also registered in Term X-3.
In Term-3, where the prior term is filtered out, empty

This is a chart expression. What I have below is returning the current distinct count, including students who were not active in the previous 'active' term.

count(distinct
{<STUDENT_ID=
p(
{<TERM_ID={"=ABOVE(TERM_ID)"}>} STUDENT_ID
)
,REG_STATUS={'Active'}
>}
STUDENT_ID)

The formula above calculates to the same value as Count(distinct{<REG_STATUS={"Active"}>} STUDENT_ID).

How do I count the overlapping students in each term?