age_ > and age_ > true at we define two agesage_ and age_ at we check whether both ages are or older the test on the left passesbut the test on the right failsso the overall conditional expression evaluates to false at we change age_ to the value of age_ is now greater than so both individual tests passcausing the overall conditional expression to evaluate as true to improve readabilityyou can use parentheses around the individual testsbut they are not required if you use parenthesesyour test would look like this(age_ > and (age_ > using or to check multiple conditions the keyword or allows you to check multiple conditions as wellbut it passes when either or both of the individual tests pass an or expression fails only when both individual tests fail let' consider two ages againbut this time we'll look for only one person to be over age_ age_ age_ > or age_ > true age_ age_ > or age_ > false we start with two age variables again at because the test for age_ at passesthe overall expression evaluates to true we then lower age_ to in the test at wboth tests now fail and the overall expression evaluates to false checking whether value is in list sometimes it' important to check whether list contains certain value before taking an action for exampleyou might want to check whether new username already exists in list of current usernames before completing someone' registration on website in mapping projectyou might want to check whether submitted location already exists in list of known locations to find out whether particular value is already in listuse the keyword in let' consider some code you might write for pizzeria we'll make list of toppings customer has requested for pizza and then check whether certain toppings are in the list