# project euler 2
↘1⍢(
  ⊂⊸(⊢+⊸⬚0↻1↙2) # generate next term from last two \
| <4e6⊢         # until we shoot over 4000000.
) [1 1]         # start with 1, 1 for fib. seq.
▽¬⊸◿₂           # filter on even.
/+              # sum.