#!/bin/bash
#=============== function 
function add
{
...  # the cmd line inspired by sequence 3  
}
#=============== main
if  ....  ; then.  # number of args is greater than 0
	for item 
	do 
	   if ....   # item is a file ?
		then
			...    # error message
			....   # stop the script with exit status to 1
	    fi
	done
	sum=...   #call add function
else
	sum=...  #call add function and use stdin
fi
echo "sum = $sum"
exit 0