Toggle a character between "T" and "F"

[Scott Waichler]

I am looking for a way to toggle the character under the cursor.  When
the character is "T", I'd like to hit a keystroke and change it to "F",
and vice versa.  I use these characters to set logical values in R
programming.  Can anyone tell me the lisp coded needed for this?

Thanks,
Scott Waichler