BASE reports: Multiple fields in header lines ?

I am building a report with multiple nested groups. There are fields that I would put together in the headers to that they are not ending up in the Detail section and get repeated with each record.
I could not find a way to put multiple fields in the header lines of a Base report.
I want the following sorting with A and B coming from the same table and only printed once per group:

A1,B1
C1
C2
C3

A2,B2…

Without multiple fields in the header I get:
A1
B1
C1
B1
C2
A2…

Instead of looking at modifying the report, maybe look at changing the Query and use CONCAT for A & B fields.

For example, current query:

 SELECT A, B, C.....

New:

   SELECT CONCAT(A, B) AS AB, C...

With A as a date and B as a name the result would be 03/01/2016Jim Smith

So change the line to

SELECT CONCAT(A, ' - ', B) AS AB, C...

and now the result is 03/01/2016 - Jim Smith

This is now a single field with all your C fields in detail section. Lots of ways you can use CONCAT.