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[Steven Eker <eker AT csl.sri.com>]

Oops! There was a typo in my previous code. Here a slightly more general 
version of the technique that runs without error in Maude 2.2:

fmod AP{X :: TRIV, Y :: TRIV} is
  sort Func{X,Y} .
  op _[_] : Func{X,Y} X$Elt -> Y$Elt [prec 17] .
endfm

fmod LIST-MAP{X :: TRIV, Y :: TRIV} is
  inc AP{X,Y} .
  pr LIST{X} .
  pr LIST{Y} .
  var E : X$Elt .
  var L : List{X} .
  var F : Func{X,Y} .
  op map : List{X} Func{X,Y} -> List{Y} .
  eq map(nil, F) = nil .
  eq map(E L, F) = F[E] map(L, F) .
endfm

fmod TEST is
  pr LIST-MAP{Nat,String} .
  pr CONVERSION .
  var N : Nat .
  op ordinal : -> Func{Nat,String} .
  eq ordinal[N] = string(N, 10) +
    if (N quo 10) rem 10 == 1 then "th"
    else
      if N rem 10 == 1 then "st"
      else if N rem 10 == 2 then "nd"
      else if N rem 10 == 3 then "rd"
      else "th" fi fi fi
    fi .
endfm

red map(1 2 3 5 7 11 13 17 19 23, ordinal) .

Steven