sS ai_weightChangeBot_theory() {
  ret [[
    import math
    
    theory validity {
      // a human weighs between 80 and 400 pounds
      $x is a human
        & $x weighs ($y pounds)
        => $y >= 80 & $y <= 400
        
      contradiction => say (That can't be right)
    }
    
    theory (weight change) {
      $x weighs ($y pounds)
        & last time $x weighed ($z pounds)
        => proc {
          if ($y < $z):
            say (That is great!)
          else if ($y > $z):
            say (Aw, too bad, tomorrow is another day.)
          else:
            say (Tomorrow will be better)
        }
    }
  
    theory (remember last weight) {
      $x weighs ($y pounds)
        => proc {
          unless contradiction:
            delete all (last time $x weighed ($z pounds))
            store      (last time $x weighed ($y pounds))
        }
    }
    
    theory (interpret input) {
      $x says (i weigh $y pounds)
        => $x weighs ($y pounds)
      
      $x says (my weight is $y pounds)
        => $x weighs ($y pounds)
    }
  ]];
}