S1←{⍵⌷⍨⍺⌷⍋⍵}

   {⍵[⍋⍵]}x
0 1 1 1 1 2 2 3 3 3 4 4 5 7 7 8 9 9 9 9 11

   0 20 10 S1⍤0 1 ⊢x
0 11 4

S←{
  s←⍵ ⍺⍺ p←⍵⌷⍨?≢⍵
  ⍺<  +/b←0>s:⍺ ∇ b⌿⍵
  ⍺<m←+/b←0≥s:p
  (⍺-m) ∇ (~b)⌿⍵
}

   0 20 10 -S⍤0 1 ⊢x
0 11 4