enters-a-copy-whose-argument-reads-to-no-bare-symbol-when-deferring.yaml

# SPDX-FileCopyrightText: Copyright (c) 2025 Objectionary.com
# SPDX-License-Identifier: MIT
---
# yamllint disable rule:line-length
# An 'unfold' line that says 'defer: true' defers only a copy one of whose
# arguments reads to a bare symbol (#1892, #1921). Here 'Φ.items.tail' reads
# to an object that knows its 'next', so the line enters the copy (#1885): it
# morphs its φ and fires what it comes to, and the 'join' line pairs two
# symbols. No 'deferred' line is written and no symbol is minted for the copy.
symbolic: |
  - λ: L_fork
    morph:
      𝑛1: $.a
      𝑛2: $.b
    unfold:
      𝑛3: {of: 𝑛2, defer: true}
    join:
      𝑛4: [𝑛1, 𝑛3]
    𝑛: ⟦ φ ↦ 𝑛4 ⟧
location: 'Q.y'
input: '⟦ step(tup) ↦ ⟦ φ ↦ tup.next ⟧, items ↦ ⟦ tail ↦ ⟦ next ↦ ⟦ φ ↦ ⟦ λ ⤍ 𝜎1 ⟧ ⟧ ⟧ ⟧, y ↦ ⟦ a ↦ ⟦ λ ⤍ 𝜎2 ⟧, b ↦ Φ.step( tup ↦ Φ.items.tail ), λ ⤍ L_fork ⟧.φ ⟧'
result: '𝜎3:λ'
protocol: |
  𝕄(Φ.y):
    𝔼(L_fork):
      𝑛1·1 := 𝜎2:λ  # 𝕄(ξ.a)
      𝕄(Φ.a🌵1):
        𝑛·1·1 := Φ.step( tup ↦ Φ.items.tail )
      𝑛2·1 := 𝑛·1·1  # 𝕄(ξ.b)
      𝑛3·1 := 𝜎1:λ:φ  # 𝕄(𝑛2.φ)
      𝑛3·1 := 𝑛3·1  # 𝔼(𝑛2)
      𝔻(𝜎3:λ) ∈ { 𝔻(𝜎2:λ), 𝔻(𝜎1:λ) }
      𝑛4·1 := 𝜎3:λ  # [𝑛1, 𝑛3]
      𝑛·1·2 := 𝜎3:λ:φ  # 𝑛
      𝑛·1·3 := 𝜎3:λ:φ  # 𝕄(𝑛·1·2)