position should be allowed to be empty. Often this value is  
being calculated and the static type of the calculation will result in an 
optionally empty type. Proposed rewording:

15.1.8 fn:remove
fn:remove($target as item()*, $position as xs:integer?) as item()*
Summary: Returns a new sequence constructed from the value of $target with the 
item at the position specified by the value of $position removed.

If $position is the empty sequence, the error FORG--99 is raised.

If $position is less than 1 or greater than the number of items in $target, 
$target is returned. Otherwise, the value returned by the function consists of 
all items of $target whose index is less than $position, followed by all items 
of $target whose index is greater than $position. If $target is the empty 
sequence, the empty sequence is returned.