Comparing and implementing calculi of explicit substitutions with eta-reduction

After defining the eta-reduction rule in the suspension calculus, and after comparing these three calculi of explicit substitutions (all with an eta rule), we then concentrate on the implementation of the rewrite rules of eta-reduction in these calculi. We note that it is usual practice when implementing the eta rule for substitution calculi, to mix isolated applications of eta-reduction with the application of other rules of the corresponding substitution calculi. The main disadvantage of this practice is that the eta rewrite rules so obtained are unclean because they have an operational semantics different from that of the eta-reduction of the λ-calculus. For the three calculi in question enlarged with adequate eta rules we show how to implement these eta rules. For the λse we build a clean implementation of the eta rule and we prove that it is not possible to follow the same approach for the λσ and λSUSP.