The following function is used for calculating the sum of the values in the nodes by iterator. Please fill the blank $($no space$)$:

int sum$($const node$*$ head$\_$ptr$)$

$\{$

const$\_$node$\_$iterator start$($head$\_$ptr$)$;

const$\_$node$\_$iterator finish;

const$\_$node$\_$iterator position;

int sum $=0$;

for $($position $=$ start; position $!=$ finish; $++$position$)$

$\{$

sum $+=$ ;

$\}$

return sum;

$\}$