If your spec contains a definition of the form
x == CHOOSE b : b \notin S
then when you create a model for it, the Toolbox will add to the model the appropriate
incantation to substitute for x a model value named "x". You could therefore write
x == CHOOSE a : a \notin {}
y == CHOOSE a : a \notin {x}
z == CHOOSE a : a \notin {x,y}
and you won't have to do any instantiation of constants.
Leslie