Function sumo#back_fn (sumo#self_connected_object -> sumo#self_connected_object)  a function that maps an object to the side that is opposite the front_fn of the object; note that this is a partial function, since some objects do not have sides, e.g., apples and spheres; note too that the range of this function is indefinite in much the way that immediate_future_fn and immediate_past_fn are indefinite; although this indefiniteness is undesirable from a theoretical standpoint, it does not have significant practical implications, since there is widespread intersubjective agreement about the most common cases
  type:  pm#unary_function_type  class of functions requiring a single argument; if R is functional, then if R(x -> y) and P(x -> z) then y=z
  type:  pm#partial_valued_relation_type  relation type that is not a total_valued_relation_type, i.e. just in case assigning values to every argument position except the last one does not necessarily mean that there is a value assignment for the last argument position; note that, if a sumo#relation is both a partial_valued_relation and a single_valued_relation, then it is a partial function
  type:  pm#irreflexive_relation_type  r is irreflexive if r(?i,?i) holds for no value of ?i
  type:  pm#asymmetric_relation_type  an antisymmetric and irreflexive relation
  type:  pm#spatial_relation_type  the class of relations that are spatial in a wide sense, e.g., mereological relations and topological relation
  supertype:  pm#spatial_relation_from_entity_with_spatial_feature (sumo#object,*)
     supertype:  pm#relation_from/to_thing_of_common_kind (*)  this type permits to categorize relations according to their signatures and hence
          offers (i) a concise way to set essential exclusion relations, and (ii) a systematic and easy-to-follow categorization
        supertype:  pm#relation__related_thing__relatedthing___related_with (*)  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type
  supertype:  pm#asymmetric_relation (?,?)  this category only serves structuration purposes: it is instance of pm#asymmetric_relation_type which is not instance of pm#class_of_inheritable_relation_type
     supertype:  pm#irreflexive_relation__irreflexiverelation (?,?)  this category only serves structuration purposes: it is instance of pm#irreflexive_relation_type which is not instance of pm#class_of_inheritable_relation_type
        supertype:  pm#binary_relation_with_particular_mathematical_property (?,?)
           supertype:  pm#relation_with_particular_mathematical_property (*)
              supertype:  pm#relation_with_particular_property (*)  this rather fuzzy type permits to group categorization schemes less common than those covered by the previous sibling categories
                 supertype:  pm#relation__related_thing__relatedthing___related_with (*)  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type
     supertype:  pm#antisymmetric_relation__antisymmetricrelation (?,?)  this category only serves structuration purposes: it is instance of pm#antisymmetric_relation_type which is not instance of pm#class_of_inheritable_relation_type
        supertype:  pm#binary_relation_with_particular_mathematical_property (?,?)


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