$ 0 3 6 9 12 15
$ 1 3 6 10 15 21
$ 10 13 16 21 30 45
TestInput ←

Parse ← ⊜(⊜⋕≠@ .)≠@\n.

Differences ← ⍢(⊂:□≡/-◫2°□⊢⇌.)(¬≍⊚⧻.°□⊢⇌)¤□
NextElement ← /+≡(⊢⇌)Differences
PrevElement ← /-⇌≡⊢Differences

/+≡NextElement Parse TestInput
/+≡PrevElement Parse TestInput

/+≡NextElement Parse &fras "day09.in"
/+≡PrevElement Parse &fras "day09.in"