code: EdgeLabel loadSqlgSchemaEdgeLabel(String edgeLabelName, VertexLabel inVertexLabel, Map<String, PropertyType> properties) {
          Preconditions.checkState(this.schema.isSqlgSchema(), "loadSqlgSchemaEdgeLabel must be called for \"%s\" found \"%s\"", SQLG_SCHEMA, this.schema.getName());
          EdgeLabel edgeLabel = EdgeLabel.loadSqlgSchemaEdgeLabel(edgeLabelName, this, inVertexLabel, properties);
          this.outEdgeLabels.put(this.schema.getName() + "." + edgeLabel.getLabel(), edgeLabel);
          inVertexLabel.inEdgeLabels.put(this.schema.getName() + "." + edgeLabel.getLabel(), edgeLabel);
          return edgeLabel;
      }
docs: Called from {@link Topology#Topology(SqlgGraph)}s constructor to preload the sqlg_schema.
  This can only be called for sqlg_schema.
  
  @param edgeLabelName The edge's label.
  @param inVertexLabel The edge's in {@link VertexLabel}. 'this' is the out {@link VertexLabel}.
  @param properties    A map of the edge's properties.
  @return The {@link EdgeLabel} that been loaded.