anchor.diff:  new(Attrs) ->
       Ts = proplists:get_value(ts, Attrs),
       Key = proplists:get_value(key, Attrs),
  -    #pvattrs{count = 1, timespan = {{Ts, Key}, {Ts, Key}}}.
  +    #pvattrs{count = 1, timespan = new_timespan({Ts, Key})}.
   
   add(#pvattrs{count=Count0, timespan=TSp0},
       #pvattrs{count=Count1, timespan=TSp1}) ->
  
positive.code: get_timespan(#pvattrs{timespan = TSp}) -> TSp.
  
  
  %% Tests
  -ifdef(TEST).
  
  
  
  -endif.
  
negative.code: 
  -record(pvattrs,    % Per-Value Attributes
          { count     % integer
          , timespan  % {MinTK, MaxTK} where TK is {Ts, Key}
          }).
  
  %% TODO maybe require a normalized form of Attrs here
  