Can \include gregorian.ly be used for some of the score only

[Martin Baker]

I am creating a lengthy score (using \bookpart) which contains a mixture of 
chant and polyphony. If I use \include “gregorian.ly” it distorts the 
polyphony, but without it I cannot set the chant the way I want.

Is there a way to confine the \include “gregorian.ly” to just the chant 
sections? I’ve tried various options but always get an error message unless 
it’s placed at the top of the file.

Thanks.