apply IH
xs : List ℕ
x b : ℕ
bs : List ℕ
MEM : List.Mem x bs
IH : shapeProd bs > 0 → x > 0
H : shapeProd (b :: bs) > 0
⊢ x > 0
xs : List ℕ
x b : ℕ
bs : List ℕ
MEM : List.Mem x bs
IH : shapeProd bs > 0 → x > 0
H : shapeProd (b :: bs) > 0
⊢ shapeProd bs > 0