{pu : Lean.Compiler.LCNF.Purity} →
  {motive : Lean.Compiler.LCNF.Arg pu → Sort u} →
    (t : Lean.Compiler.LCNF.Arg pu) →
      t.ctorIdx = 1 → ((fvarId : Lean.FVarId) → motive (Lean.Compiler.LCNF.Arg.fvar fvarId)) → motive t