Returns single element, or null
if the sequence is empty or has more than one element.
The operation is terminal.
Returns the single element matching the given predicate, or null
if element was not found or more than one element was found.
The operation is terminal.
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Licensed under the Apache License, Version 2.0.
https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.sequences/single-or-null.html