Higher-order functions

type Program = <program>[ Day* ]
type Day = <date day=String>[ Invited? Talk+ ]
type Invited = <invited>[ Title Author+ ]
type Talk = <talk>[ Title Author+ ]
type Author = <author> String
type Title = <title> String

let patch_program 
  (p :[Program], f :(Invited -> Invited) & (Talk -> Talk)):[Program] =
  xtransform p with (Invited | Talk) & x -> [ (f x) ]

let first_author ([Program] -> [Program];
                  Invited -> Invited;
                  Talk -> Talk)
| [ Program ] & p -> patch_program (p,first_author)
| <(k)>[ t a _* ] -> <(k)>[ t a ]