SML problem with use of binary trees and pattern matching : ml

Hi,
I have got a problem with the use of pattern matching and binary trees
in SML.

When I type in:

datatype 'a btree = emptybt | build of 'a * 'a btree * 'a btree;

and then define a function with a 'a btree as argument using pattern
matching:

fun isemptybtree emptybt = true
| isemptybtree (build (v, l, r)) = false;

there is no problem. However, in my programming course we have to use a
binary tree module:

signature bintree_s = sig
type 'a bintree
exception emptytree
val emptybt: 'a bintree
val build: 'a * 'a bintree * 'a bintree -> 'a bintree
val root: 'a bintree -> 'a
val left: 'a bintree -> 'a bintree
val right: 'a bintree -> 'a bintree
val isemptybt: 'a bintree -> bool
end

structure BINTREE : bintree_s = struct
datatype 'a bintree = emptybt
| build of 'a * 'a bintree * 'a bintree
exception emptytree
fun root emptybt = raise emptytree
| root (build (x, l, r)) = x
fun left emptybt = raise emptytree
| left (build (x, l, r)) = l
fun right emptybt = raise emptytree
| right (build (x, l, r)) = r
fun isemptybt emptybt = true
| isemptybt _ = false
end


I load that module, open it, and then I want to define the same
function as above:

fun isemptybtree emptybt = true
| isemptybtree (build (v, l, r)) = false;

I get the following error:
Error: non-constructor applied to argument in pattern: build

What can I do about that and how can I use pattern matching with this
module? I'd be glad to get some hints.

Valentin

Change the signature to have the entire definition of the datatype.
Eliminate the val lines for 'emptybt' and 'build'.