please write comments to explain how this function is using a Stack!

from pythonds.basic import Stack det postfixEval(postfixExpr): operandStack = Stack() tokenlist = postfixExpr.split() for token in tokenlist: if token in "0123456789": operandStack.push(int(token)) else: operand2 = operandStack.pop() operand1 = operandStack.pop() result doMath(token, operandi, operand2) operandStack.push(result) return operandStack.pop() def domath(op, opi, op2): if op == "*": return op1 * op2 elif op return opl / op2 elif op == "+": return opi + op2 else: return op1 op 2 print(postfixEval('7 8 4 3 2 +/'))