Relation pm#where_relation__whererelation (*)  where, from/to where, ...
  subtype:  pm#from/to  pm#place  pm#path_length  pm#within_group  pm#relation_to_another_spatial_object  pm#spatial_origin
  supertype:  pm#wh-/how_relation (*)  this type permits to categorize relations according to the usual who/what/why/where/when/how questions ; this is a traditional but very subjective and ineffective way of categorizing relations 
     supertype:  pm#relation__related_thing__relatedthing___related_with (*)  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type


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