pitch trill accidentals don't apply to the rest of the bar

[Graham Percival]

This _may_ be a feature, but it looks confusing to me:

There's an fis in parenthesis, followed by an f.  The f has no natural 
sign, so if I were sight-reading, I wouldn't know if it was supposed to 
be f or f#.


\version "2.7.35"
\relative c' {
\pitchedTrill c4\startTrillSpan fis f\stopTrillSpan
}