Write a function called mycase$($chars$,$ option$)$ to return a copy of string chars with the case converted. The input argument option is a keyword $($or keyworded$)$ argument with a default value "lower". When option $=$ "lower", covert all letters to lowercase; when option $=$ "upper", covert all letters to uppercase; when option $=$ "capitalize", convert the first character to uppercase and the rest lowercase. $($Python built$-$in functions are allowed. Define the function only without calling it$.$ Assume all characters in chars are alphabetic and there is at least one character.$)$