CALC
This command lets you do calculations right in the middle of your text.  Type .
   the problem, highlight it, and use the CALC command.  Results of 
   calculations have a dotted underline to distinguish them from ordinary typed 
   text (the underline doesn't print).  You can retrieve the calculations 
   underlying the result by placing the cursor anywhere on the underlined result 
   and using the CALC command.  This is called "unpocketing."
CALC can do + (addition), - (subtraction), * (multiplication), / (division), % 
   (percent), sqrt (square root), sum (add a tabled column of numbers), 
   avg (average a tabbed column of numbers).  Use parenthesis to ensure that 
   certain operations are performed first. For example:(25%*100)*(16/4)-25 = 75 
Named variables may be used to link calculations.  For example, if you type, 
   highlight and CALC: price:5.95  , the word "price" can be used in 
   calculating various quantities: price*10 = 59.50, for example.  To change 
   "price", place the cursor on the original underlined 5.95, and use the CALC 
   command.  The variable name and number "price:5:95" pops out.  Alter the 
   number, then highlight and CALC the entire resulting expression.  All 
   calculations affected by "price" will change.
Use "use( )"  to identify the position of entries in a table by column and 
   row (relative to the column and row of the calculation).  For example, 
   use(2-1) * use(3 0) means "multiply the number in the 2nd column to the 
   right and the first row up by the number in the 3rd column in the same row."

[UNDO] has no effect on calculations..