Could somebody give full code for a macro that would do this:

- By clicking a cell in a range of certain defined cells the cell
would be emptied and then a certain letter, e.g. the letter 'A', would
be defined as the value of the cell if the cell did not already have
the letter as its value;

- By clicking a cell in a range of the certain defined cells the cell
would be emptied if the cell already had the letter as its value.

Thanks!