Speaking of line noise... (continued)

(~R∈R○.×R)/R←↓⍳R

R ∈ … : for each element, is it in the table?

0 0 1 0 1 ...

~ … : invert that list.

1 1 0 1 0 ...

… / R : pick all the elements in R that have a defined value in their position in the new list.

2 3 5 ...