Explicit substitutions

References

• Lescanne, Rouyer-Degli, "The Calculus of explicit substitutions lambda-upsilon"
• https://github.com/maciej-bendkowski/combinatorics-of-explicit-substitutions/
• https://github.com/cls/strong-krivine
• Abadi, Cardelli, Curien, Levy, "Explicit substitutions"
• Kluge, "Abstract Computing Machines" (sec 6.3)
• Archambault-Bouffard, Monnier, "Implementation of Explicit Substitutions: from λσ to the Suspension Calculus"