Left side of mutating operator isn't mutable: 'self' is immutable code example
Example: Left side of mutating operator isn't mutable: 'self' is immutable
mutating func nextQuestion(){ // add "mutating" before func to change value of variables from (the struct)
if questionNumber + 1 < questions.count { // to count answered questions
questionNumber += 1
}else{
questionNumber = 0
}
}