startingLoc and length should be allowed to be empty. Often these values are 
being calculated and the static type of the calculation will result in an 
optionally empty type. Proposed rewording:

fn:subsequence($sourceSeq as item()*, $startingLoc as xs:double?) as item()*
fn:subsequence( $sourceSeq  as item()*, 
$startingLoc  as xs:double?, 
$length  as xs:double?) as item()* 

Summary: Returns the contiguous sequence of items in the value of $sourceSeq 
beginning at the position indicated by the value of $startingLoc and 
continuing for the number of items indicated by the value of $length. 

If $startingLoc is the empty sequence, the error FORG--99 is raised.
If $length is the empty sequence, the result is the empty sequence.

More specifically, returns the items in $sourceString whose position $p obeys:

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