Core_kernel.Bounded_indexBounded_index creates unique index types with explicit bounds and human-readable labels. "(thing 2 of 0 to 4)" refers to a 0-based index for the third element of five with the label "thing", whereas a 1-based index for the second element of twelve with the label "item" renders as "(item 2 of 1 to 12)", even though both represent the index 2.
Use Bounded_index to help distinguish between different index types when reading rendered values, deserializing sexps, and typechecking. Consider using Bounded_index to label fixed pools of resources such as cores in a cpu, worker processes in a parallel application, or machines in a cluster.