“…However, while algebras and coalgebras do not completely constrain each other, and varying the algebras allows capturing the linear time/branching time spectrum, they were specified within the same category, with their semantical connection hardwired through distributivity requirements precluding, e.g., capturing the language hierarchies. On the other hand, semantic connections of algebras and coalgebras were studied in a wide variety of frameworks, and by a wide variety of techniques [25,21,23, to mention just a few], and it is possible that the language hierarchies, language acceptance, and computability and complexity concepts could have been captured in that framework if the community moved in that direction. The language acceptance relation and the trace equivalences have in fact been captured in [19], but by combining coalgebras and monads, which seem to indirectly capture the underlying semantical connection.…”