Relation sumo#reciprocal_fn ()
  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#total_valued_relation_type  when there exists an assignment for the last argument position of the relation given any assignment of values to every argument position except the last one; note that declaring a relation to be both a total_valued_relation and a single_valued_relation means that it is a total function
  type:  pm#type_of_relation_extended_to_quantities  relation that, when it is true on a sequence of arguments that are real_numbers, it is also true on a sequence of constant_quantites with those magnitudes in some unit of measure; for example, the less_than relation is extended to quantities; this means that for all pairs of quantities ?q1 and ?q2, [?q1,  sumo#less_than: ?q2] if and only if, for some numbers ?n1 and ?n2 and unit ?u, [q1 =  sumo#measure_fn(?n1,?u)], [q2 =  sumo#measure_fn(?n2,?u)] and [?n1, less_than: ?n2] for all units on which ?q1 and ?q2 can be measured; note that, when a relation_extended_to_quantities is extended from real_numbers to constant_quantities, the constant_quantities must be measured along the same physical dimension