abstract fun add(element: E): Boolean

For Common, JS

Adds the specified element to the collection.

Return true if the element has been added, false if the collection does not support duplicates and the element is already contained in the collection.

For JVM

Adds the specified element to the collection.

This method is redeclared as abstract, because it's not implemented in the base class, so it must be always overridden in the concrete mutable collection implementation.

Return true if the element has been added, false if the collection does not support duplicates and the element is already contained in the collection.