code: public JTSGeometryExpression<Geometry> envelope() {
          if (envelope == null) {
              envelope = JTSGeometryExpressions.geometryOperation(SpatialOps.ENVELOPE, mixin);
          }
          return envelope;
      }
docs: The minimum bounding box for this Geometry, returned as a Geometry. The
  polygon is defined by the corner points of the bounding box [(MINX, MINY), (MAXX, MINY), (MAXX, MAXY),
  (MINX, MAXY), (MINX, MINY)]. Minimums for Z and M may be added. The simplest representation of an
  Envelope is as two direct positions, one containing all the minimums, and another all the maximums. In some
  cases, this coordinate will be outside the range of validity for the Spatial Reference System.
  
  @return envelope