code: public EdgeLabel ensureEdgeLabelExist(final String edgeLabelName, final VertexLabel inVertexLabel, Map<String, PropertyType> properties) {
          return this.getSchema().ensureEdgeLabelExist(edgeLabelName, this, inVertexLabel, properties);
      }
docs: Ensures that the {@link EdgeLabel} exists. It will be created if it does not exists.
  "this" is the out {@link VertexLabel} and inVertexLabel is the inVertexLabel
  This method is equivalent to {@link Schema#ensureEdgeLabelExist(String, VertexLabel, VertexLabel, Map)}
  
  @param edgeLabelName The EdgeLabel's label's name.
  @param inVertexLabel The edge's in VertexLabel.
  @param properties    The EdgeLabel's properties
  @return The EdgeLabel