I figured it out. The following introduction of order and order_type  
[2] triggers the error I was seeing previously [1]. This should be  
enough to reproduce the issue and, hopefully, provide clues on how to  
fix it. I have no clues, btw.

	Thanks, Joel

[1] Error

ocamlbuild c.byte
+ /usr/local/bin/ocamlc.opt -c -I +camlp4 -pp 'camlp4of -I src - 
filter map -filter fold -filter trash ' -o b.cmo b.ml
File "ghost-location", line 1, characters 0-0:
This expression has type
   < array : 'b. ('a -> 'b -> 'a) -> 'b array -> 'a; bool : bool -> 'a;
     expr : A.expr -> 'a; float : float -> 'a; id : A.id -> 'a;
     input_decl : A.input_decl -> 'a; int : int -> 'a;
     list : 'c. ('a -> 'c -> 'a) -> 'c list -> 'a;
     option : 'd. ('a -> 'd -> 'a) -> 'd option -> 'a; order :  
A.order -> 'a;
     order_type : A.order_type -> 'a; program : A.program -> 'a;
     ref : 'e. ('a -> 'e -> 'a) -> 'e ref -> 'a;
     statement : A.statement -> 'a; string : string -> 'a;
     subscript : A.subscript -> 'a; ty : A.ty -> 'a; .. >
   as 'a
but is here used with type A.order_type

[2] Code with order and order_type.

type id = string

and program = statement list

and ty =
     [
     | `TyAny
     ]

and statement =
     [
     | `Skip
     | `InputDecls of input_decl list
     ]

and subscript = expr list

and input_decl =
     [
     | `InputDecl of id * ty * expr
     | `FunArgDecl of id * ty * subscript
     ]

and expr =
     [
     | `Int of int
     | `Float of float
     | `Str of string
     ]

and order = {
   order_type: order_type;
}

and order_type =
     [
     | `Buy
     | `Sell
     | `Short
     | `Cover
     ]



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