$```$

$\}$

$//$ Implementation for the MathStack class

$//*******************************************************$

$//$ Member function add. add pops

$//$ the first two values off the stack and

$//$ adds them. The sum is pushed onto the stack.

$//***************************************************$

void MathStack::add$()$

$\{$

int num, sum;

$//$ Pop the first two values off the stack.

pop$($sum$)$;

pop$($num$)$;

$//$ Add the two values, store in sum.

sum $+=$ num;

$//$ Push sum back onto the stack.

push$($sum$)$;

$\}$

$``````$

$//**********************************************************$

$//$ Member function sub. sub pops the $*$

$//$ first two values off the stack. The $*$

$//$ second value is subtracted from the $*$

$//$ first value. The difference is pushed $*$

$//$ onto the stack.

$//******************************************************$

void MathStack::sub$()$

$\{$

int num, diff;

$//$ Pop the first two values off the stack.

pop$($diff$)$;

pop$($num$)$;

$//$ Subtract num from diff.

diff $-=$ num;

$//$ Push diff back onto the stack.

push$($diff$)$;

$\}$

$//$ TO DO $4$: Add below the implementation for the multiply function

$```$