Plan A: ontology-agnostic

key/value pairs:

location => moon
unit => ppm
shoesize => 42
shoesize => 42^^xsd:decimal

can be attached to value

      <    undef { valid => "not really" } if n.suspicious_device 
                                           && [n] > 100 m
        or [n]   { valid => "looks good" } otherwise 
      >