/guava/guava/src/com/google/common/collect/Synchronized.java
if ( ( keys) == null) {   keys = multiset( delegate( ).keys( ) , mutex) ; }
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} @CheckForNull transient NavigableSet<K> navigableKeySet; @Override public NavigableSet<K> navigableKeySet() { synchronized (mutex) { if (navigableKeySet == null) { <extra_id_0> } return navigableKeySet; } } @Override @CheckForNull