[FO] fn:substring arguments

Hello,
   The F&O spec defines the fn:substring function as,

fn:substring($sourceString  as xs:string?,
                  $startingLoc  as xs:double) as xs:string

In the above method signature, the 2nd argument is a xs:double.

Whereas in Java language, substring function of the String library
accepts an "int" argument.

What is the motivation for the F&O spec to define the startingLoc
argument as, xs:double and not an integer?


-- 
Regards,
Mukul Gandhi