[F&O] rounding functions for decimal arguments

The draft says:

"For xs:float and xs:double arguments, if the argument is "NaN", "NaN"
is returned. 
Except for fn:abs(), for xs:float and xs:double arguments, if the
argument is positive or negative infinity, positive or negative
infinity is returned." 

Why not for xs:decimal too?
-- 
Colin Paul Adams
Preston Lancashire