public static ImmutableDictionary <string, T> Compose <T>(
this ImmutableDictionary <string, T> left,
ImmutableDictionary <string, T> right
) where T : IUnifiable <T>
=> left
.Select(kvp => KeyValuePair.Create(kvp.Key, right.Apply(kvp.Value)))
.Where(kvp => !(kvp.Value is Variable v && v.Name == kvp.Key))