The definition of a user-defined function in 4.5 Function Declarations has two flaws:

1. It's not clear whether the function body is the enclosed expression or the expression it contains.
2. It is not similar to the definition of external functions.

Here are the current definitions:

[Definition: For a user-defined function, the function declaration includes an expression called the function body that defines how the result of the function is computed from its parameters.]. The static context for a function body includes all functions that are declared or imported anywhere in the Prolog, but it includes only those variables and namespaces that are declared or imported earlier in the Prolog than the function that is being defined.

[Definition: External functions are functions that are implemented outside the query environment.] For example, an XQuery implementation might provide a set of external functions in addition to the core function library described in [XQuery 1.0 and XPath 2.0 Functions and Operators]. External functions are identified by the keyword external. The purpose of a function declaration for an external function is to declare the datatypes of the function parameters and result, for use in type checking of the query that contains or imports the function declaration.

I think this could be improved by adding a production for FunctionBody and rephrasing the definition:
 	
<proposed>
FunctionDecl 	   ::=    	"declare" ("deterministic" | "nondeterministic")? "function" QName "(" ParamList? ")" ("as" SequenceType)? (FunctionBody | "external")

ParamList 	   ::=    	Param ("," Param)*

Param 	   ::=    	"$" QName TypeDeclaration?

TypeDeclaration 	   ::=    	"as" SequenceType

FunctionBody ::= EnclosedExpr


[Definition: User defined functions are functions that contain a function body, which provides the implementation of the function using an XQuery expression.]