Williamson used the word cassation in the sense of a miniature opera including audience participation.
Williamson had a great deal of success with these cassations, which have had performances in Britain, Australia, France, the US, and in hospitals in Tanzania and Zambia.
These include a Strindberg-based opera Easter (with a libretto by Myfanwy Piper), a Symphony No.
8 Agamemnon (based on the poem by Dame Iris Murdoch), and sketches for a Piano Concerto No.
5, which he had hoped to write for his Australian friend, the pianist Antony Gray.