Works licensed under the GFDL may be included in with non-GFDL-licensed works only if it is clear which parts of the work are licensed as the GFDL. For example, in a book of poetry it would be easy to label some poems as licensed under the GFDL and some not licensed under it. But it would not be easy to label if part of a song was licensed as GFDL and the rest was not, so this would not be allowed. 