List.swap_head: def of bendlib-kernel-list@1.0.0.0
Import: import bendlib-kernel-list@1.0.0.0/list.bend as List
Lines 5-15 of bendlib-kernel-list@1.0.0.0/list.bend: https://jend.leeeet.dev/read.txt?path=bendlib-kernel-list@1.0.0.0/list.bend&start_line=5&end_line=15

import Base

# Swap the first two elements (no-op on lists shorter than two).
def swap_head(-A: Data, xs: List<&2, A>) -> List<&2, A>:
  match xs:
    case Nil{}:
      Nil{}
    case +x <> t:
      match t:
        case Nil{}:
          x <> Nil{}
        case +y <> u:
          y <> x <> u

Uses (1):
  https://jend.leeeet.dev/definition.txt?path=Base/base.bend&name=List