Parametricity states that polymorphic functions behave the same regardless of how they are instantiated. When developing polymorphic programs, Wadler’s free theorems [Wadler 1989] can serve as free specifications, which can turn otherwise partial specifications into total ones, and can make otherwise realizable specifications unrealizable. This is of particular interest to the field of program synthesis, where the unrealizability of a specification can be used to prune the search space. In this paper, we focus on the interaction between parametricity, input-output examples, and sketches. Unfortunately, free theorems introduce universally quantified functions that make automated reasoning difficult. Container morphisms provide an alternative representation for polymorphic functions that captures parametricity in a more manageable way [Abbott et al. 2003]. By using a translation to the container setting, we show how reasoning about the realizability of polymorphic programs with input-output examples can be automated.