[XeTeX] Automatic glyph substitution

[Gareth Hughes]

Hi all!

I am using Gill Sans MT for headings in a document that I'm setting. The
font I'm using does not automatically substitute the ligature glyphs
'fi' and 'fl' that are supplied with the typefaces. Without editing the
font files, is there an easy way to make the substitution automatic
within the document? I know that I can use the \namedglyph command
sequence with xltxtra, but I am hoping for something more graceful.

Thanks,

Gareth.