My answer is really summed up in the comment above. As I was about to say when I reached the limit -

I want Libre-office to respond to me as follows:

I highlight a stack of cells from row r1 to row r2, say, in column c.
(Libre-office knows the values of r1,r2, and c.)
I press ADD
a - Libre-office inserts a blank row immediately below row r2.

## b - Libre-office adds the values in column c from row r1 to row r2 and places the value in the new row r2+1

The advantage here is that I don't need to do the counting and type in a particular formula involving the number of cells over which to add. You can see that this offers a tremendous speed-up.
Alas - this is not just an application of libre-office as it stands; I am asking for libre-office
a - to mimic what I I would do with right click, insert a row
b - to analyze the pattern of selected cells in order to discover r1,r2, and c. ( Libre-office must 'know' these values in order to maintain the display of he selected cells.)
c - to create the right formula involving SUM and the integer 1 + r2 - r1; this is the number of values to be added.

## d - to execute this formula

I am a bit hazy about how to ensure that the answer falls in the cell in the new row 1+r2 and column c
However I am sure that someone can enlighten me!