Platform and version requirements: JVM (1.0), Native (1.3)

abstract fun add(element: E): Boolean

For Common, Native

Adds the specified element to the set.

Return true if the element has been added, false if the element is already contained in the set.

For JVM

Adds the specified element to the set.

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 element is already contained in the set.