code: public GeometryExpression<Geometry> union(Expression<? extends Geometry> geometry) {
          return GeometryExpressions.geometryOperation(SpatialOps.UNION, mixin, geometry);
      }
docs: Returns a geometric object that represents the Point set
  union of this geometric object with anotherGeometry.
  
  @param geometry other geometry
  @return union of this and the other geometry