Final answer:
In 'Dream of a Witches' Sabbath,' Berlioz's use of 'Dies irae' exemplifies the practice of quotation or thematic transformation in classical music composition.
Step-by-step explanation:
In Hector Berlioz's 'Dream of a Witches' Sabbath' from his Symphony Fantastique, the incorporation of 'Dies irae' is an example of the compositional practice known as quotation or thematic transformation. 'Dies irae' is a Gregorian chant from the Mass for the Dead, and its inclusion within a new musical context creates an intertextual reference that enriches the character and meaning of Berlioz's piece. This practice is quite common in classical music, where composers often borrow musical ideas or motifs from existing works and transform them to serve new expressive purposes.