Julian Rushton has suggested that any solution should satisfy five criteria: a "dark saying" must be involved; the theme "is not played"; the theme should be "well known" (as Elgar stated multiple times); it should explain Elgar's remark that Dora Penny should have been, "of all people", the one to solve the Enigma; and fifthly, some musical observations in the notes Elgar provided to accompany the pianola roll edition may be part of the solution. Furthermore, the solution (if it exists) "must be multivalent, must deal with musical as well as cryptographic issues, must produce workable counterpoint within Elgar's stylistic range, and must at the same time seem obvious (and not just to its begetter)".