### Question:
GraphBlock implements ContainerBlock { @Override public BooleanExpression exists() { return BooleanOperation.create(Ops.EXISTS, this); } GraphBlock(Expression<UID> context, List<Block> blocks, Predicate... filters); @Override Predicate not(); @Override @SuppressWarnings("unchecked") R accept(Visitor<R, C> v, C context); @Override Class<? extends Boolean> getType(); List<Block> getBlocks(); @Nullable Predicate getFilters(); Expression<UID> getContext(); @Override BooleanExpression exists(); @Override int hashCode(); @Override boolean equals(Object o); @Override String toString();  }### Answer:
@Test public void Exists() { Block block = Blocks.graphFilter(new QNODE<UID>(UID.class, "c"), Blocks.SPO); assertEquals("exists GRAPH {c}{ {s} {p} {o} . }", block.exists().toString()); }