(if arity_of_x = None then failwithProblem p "step on var non occurring in problem");
(if Util.option_get(arity_of_x) = min_int then failwithProblem p "step on fake variable");
(if Util.option_get(arity_of_x) <= 0 then failwithProblem p "step on var of non-positive arity");
(if arity_of_x = None then failwithProblem p "step on var non occurring in problem");
(if Util.option_get(arity_of_x) = min_int then failwithProblem p "step on fake variable");
(if Util.option_get(arity_of_x) <= 0 then failwithProblem p "step on var of non-positive arity");