indexOf
Returns first index of element, or -1 if the sequence does not contain element.
fun <T> Sequence<T>.indexOf(element: T): Int(source)
val fruits = sequenceOf("apple", "banana", "cherry", "banana")
// Find first occurrence of "banana"
println(fruits.indexOf("banana")) // prints 1
// Find an element that isn't present
println(fruits.indexOf("orange")) // prints -1