為 \beginsection 巨集取得正確的分頁符

Question

我在問題中找到了這段程式碼Plain TeX 中某部分的第一段沒有縮排

\def\mysection#1{%
  \goodbreak
  \vskip2\baselineskip
  \noindent #1\par
  \nobreak
  \vskip\baselineskip
  \everypar={{\setbox0\lastbox}\everypar={}}}

但當頁面在新段落上中斷時，它不會拉伸頁面底部（此處拉伸是在章節標題周圍的跳過中發現的）。

所以我嘗試了

\def\mysection#1{%
  \filbreak
  \vskip2\baselineskip
  \noindent #1\par
  \nobreak
  \vskip\baselineskip
  \everypar={{\setbox0\lastbox}\everypar={}}}

現在鼓勵分頁太多了...（第二頁在 3d 部分之前就被破壞了）

這是我的問題：我該如何定義\mysection一個

• 當（且僅當）章節標題和該章節第一段的第一行沒有空格時，才會中斷頁面；
• \hfil如果頁面在章節標題上被破壞，則以 a 結束頁面？

測試文件：

\def\mysection#1{%
    \filbreak
    %\goodbreak
    \vskip2\baselineskip
    \noindent #1\par
    \nobreak
    \vskip\baselineskip
    \everypar={{\setbox0\lastbox}\everypar={}}}

some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text 

\mysection{Hello world!}
some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text 

\mysection{Nice title!}
some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text 

some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text 

some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text 

\mysection{This one too}
some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text 

some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text 
\bye

Answer 1

使用與 相同的想法\filbreak，但具有有限plus分量：

\def\mysection#1{%
    \par
    \vskip2\baselineskip plus 3\baselineskip
    \penalty-500
    \vskip 0pt plus -3\baselineskip
    \noindent #1\par
    \nobreak
    \vskip\baselineskip
    \everypar={{\setbox0\lastbox}\everypar={}}}

some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text 

\mysection{Hello world!}
some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text 

\mysection{Nice title!}
some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text 

some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text 

some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text 

\mysection{This one too}
some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text 

some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text some text 

\bye

有什麼想法\filbreak？這個命令發出兩個\vskip命令，\penalty中間有一個：

\vskip<specs-a>
\penalty<amount>
\vskip<specs-b>

有以下三種情況：

1. 這些項目不分頁

2. 在第一個膠水處進行分頁

3. 罰分頁

請注意，第二個黏合點不是可行的分頁點，因為它前面有一個懲罰，這是一個可丟棄的項目。

在情況 1 中，兩種膠水將出現在頁面上；在情況 2 中，這三個項目將在分頁符號處消失；在情況 3 中，第二個膠水項將會消失。

現在，做什麼呢\filbreak？它的定義是

\par\vfil\penalty-200\vfilneg

其中\vfil和\vfilneg是原語等價於

\vskip 0pt plus 1fil

和

\vskip 0pt plus -1fil

分別。因此，在情況 1 中，不會產生垂直空間，因為無限分量相互抵消。在情況 2 中，什麼也看不見。在情況 3 中，只有plus 1fil會起作用，為頁面添加無限的可拉伸性。

在我的變體中，我使用與 (-500) 相同的懲罰\goodbreak，但第一個黏合項是2\baselineskip plus 3\baselineskip，第二個黏合項是0pt plus -3\baselineskip。想法是相同的：在情況 1 中，膠水的淨量為2\baselineskip；在情況 2 中什麼也看不見；在情況 3 中，拉伸量為3\baselineskip，因此 TeX 將被要求不要留下超過容納三行所需的空間：節標題、其後的空行以及下一段的第一行。老實說，我會嘗試使用兩條線，以避免俱樂部線。