USING: locals ; :: next-fib-seq ( seq -- newseq ) [let | next-term [ seq 2 tail* sum ] | seq next-term suffix ] ; :: fib-seq-exceed? ( seq -- ? ) seq peek 4000000 > ; :: fib-seq ( seq -- newseq ) [let | next [ seq next-fib-seq ] | next fib-seq-exceed? [ seq ] [ next fib-seq ] if ] ; : solve ( -- ) { 1 2 } fib-seq [ even? ] filter sum .

Done