Primitive for retrieving the upper and lower bounds of Var. Lo and Hi return the minimum and maximum (respectively) of the variable's interval. If the variable is integer and a bound is finite, then that bound will be returned as an integer; in all other cases it will be returned as a float. If Var has not been declared before, it will be turned into an unrestricted real variable. If Var is a ground number, Lo and Hi will give appropriate bounds based on the type and value of Var: exact bounds for floats, bounded reals and integers, and best safe approximations for rationals.