import java.time.Instant

fun main() {
    // Check if a is in abc
    println('a' in "abc")
    // Check if a is not in abc
    println('a' !in "abc")
    // Check if now is within the bounds, using comparable under the hood
    println(Instant.now() in Instant.MIN..Instant.MAX)
}

/// Playground
fun isValidIdentifier(s: String): Boolean {
    fun isValidCharacter(ch: Char): Boolean = ch == '_' || ch.isLetterOrDigit()
    if (s.isEmpty() || s[0].isDigit()) {
        return false
    }
    for (ch in s) {
        if (!isValidCharacter(ch)) {
            return false
        }
    }
    return true
}