PostGIS  2.3.7dev-r@@SVN_REVISION@@
PG_FUNCTION_INFO_V1 ( LWGEOM_collect  )

collect( geom, geom ) returns a geometry which contains all the sub_objects from both of the argument geometries

Returns
geometry is the simplest possible, based on the types of the collected objects ie. if all are of either X or multiX, then a multiX is returned.