In The Annotated Snark, Martin Gardner, the editor and annotator, published the full text of Lewis Carroll’s “The Hunting of the Snark” with the original illustrations by Henry Holiday. Although no-one knows what a Snark looks like we do know that the Snark is a Boojum. But no-one knows what a Boojum looks like either. On page 19 of the Penguin 1967 edition we see the suppressed Boojum, illustrated by Holiday but not seen until 1932. No one knows what a Soft Brexit looks like. So, for the time being, I think this illustration will do.