add the text abstract I wrote for submission

abstract.md

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+The [Inferno](https://gitlab.inria.fr/fpottier/inferno) library uses an
+applicative functor `'a co` to represent constraint-based
+inference problems that elaborate into explicitly-typed term
+representations. A central operation of the Inferno API is the
+`exist` quantifier, which generates a fresh inference
+variable scoped in an existential constraint. `exist`
+returns the result of solving the constraint, as well as the
+elaborated type inferred for the inference variable:
+
+```
+val exist : (variable -> 'a co) -> ('a * ty) co
+```
+
+We found that programming with this interface is difficult and leads
+to program that are hard to read. The difficulty is specific to
+applicative functors, there would be standard solutions with a monad.
+
+We report on our API design work to solve this issue. We start by
+explaining the programming difficulties and why the Inferno
+functor is not a monad, then a first experiment with a codensity
+monad, and finally a simpler solution that avoids mixing several
+functors in the same codebase. Our approach makes good use of
+the "binding operators" recently introduced in OCaml to provide
+flexible syntactic sugar for monad or applicative-like structures.
+
+We believe that the API design pattern we propose, "turning direct
+outputs into modal inputs", may apply to any applicative functors
+with quantifiers. It may already be in use, and its discussion may
+benefit other projects. Experience sharing with the ML Workshop
+audience could be very valuable.