Slur over single note?

[Joel C. Salomon]

Still from the John Crook’s Peter Pan score project, but a different
piece (“The Flying Away”, page 12 in the score at
<http://hdl.handle.net/1802/24425>):

As show in the attached image, there seems to be a slur drawn over a
single whole-note.  It’s very clearly over that note, not over the other
voice in that measure—which doesn’t seem reasonable to me.

Can someone explain to me what’s going on there, and how to achieve this
in LilyPond?  I was thinking to use something like

    \graceAfter a1(  { s32) }

but if this is a semi-standard musical construction I’m unfamiliar with,
there might be a better way to code this.

—Joel C. Salomon

slur over single note.png
Description: PNG image