@Override
        public Object apply(final Object object) {
            if (object instanceof Map)
                return new LinkedHashSet<>(((Map<?,?>) object).keySet());
            else if (object instanceof Map.Entry)
                return ((Map.Entry) object).getKey();
            else if (object instanceof Path)
                return new ArrayList<>(((Path) object).labels());
            else
                throw new IllegalArgumentException("The provided object does not have accessible keys: " + object.getClass());
        }
The keys associated with the data structure.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/Column.java