firstOrNull
Platform and version requirements: JVM (1.0), JS (1.0), Native (1.0)
fun firstOrNull(): E?
Returns the first element, or null if this deque is empty.
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Licensed under the Apache License, Version 2.0.
https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-array-deque/first-or-null.html
