sourceIterator
Platform and version requirements: JVM (1.0), JS (1.0), Native (1.0) 
abstract fun sourceIterator(): Iterator<T>
Returns an Iterator over the elements of the source of this grouping.
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Licensed under the Apache License, Version 2.0.
    https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-grouping/source-iterator.html