slur cross staves

[Robert Garrigos]

This is the original music to copy:




I have this:

<<
    {
      \change Staff = "upper" \set tieWaitForNote = ##t
      \grace {cs'4_~ fs_~ cs'~ s4}  <cs, fs cs'>1^\fermata
    }
    \\
    {
      \set tieWaitForNote = ##t
      \grace { fs,,4~^\pp\sustainOn cs'~ as'^~ s4*4}  <fs,, fs' cs' 
as'>1_\fermata
    }
  >>

Which makes me this:



But I'm still missing the slur from cue low f to high c. Any ideas how to do it?

Thanks.

Robert