fmod WITCH is

    *** Anything can be a thing - duck, person, witch etc.
    sort Thing .
    
    *** Sorts for *particular* things
    sorts Witch Burns Wood Floats .

    *** Represents the weight of an object
    sort Weight .

    *** The subsort relationships
    subsort Witch < Burns < Wood < Floats < Thing .

    op weight : Thing -> Weight .

    *** Equality operator for Things
    op _===_ : Weight Weight -> Bool .

    *** Constant for the duck
    op duck : -> Floats .

    *** Constant for the possible witch
    op witch? : -> Thing .

    var wood : Wood .
    var burn : Burns .
    var witch : Witch .
    var floats : Floats .
    var x : Thing .
  
    *** The membership axioms now capture most of the
    *** 'logic'
    mb floats : Wood .
    mb wood : Burns .
    mb burn : Witch .
    cmb x : Floats if (weight(x) === weight(duck)) .

    eq weight(witch?) === weight(duck) = true .

endfm