public SubgraphStrategy create() {
            if (null == this.vertexCriterion && null == this.edgeCriterion && null == this.vertexPropertyCriterion)
                throw new IllegalStateException("A subgraph must be filtered by a vertex, edge, or vertex property criterion");
            return new SubgraphStrategy(this);
        }
The traversal predicate that defines the vertex properties to include in the subgraph.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/strategy/decoration/SubgraphStrategy.java