filter=: {: ,~ ({~ i.@>.&.(%&20)@#) NB. take every 20th and last item
filter evolve 'METHINKS IT IS LIKE A WEASEL'
XXURVQXKQXDLCGFVICCUA NUQPND
MEFHINVQQXT IW LIKEUA WEAPEL
METHINVS IT IW LIKEUA WEAPEL
METHINKS IT IS LIKE A WEASEL