将 ConTeXt 中的一个或多个页面切换为横向

Question

我尝试了以下代码。我不明白为什么页面大小保持不变。因此，纸张大小仍为 A4 纵向，而不是切换为横向。

\definepapersize[standing][A4]          [A4]
\definepapersize[lying]   [A4,landscape][A4,landscape]

\showframe

\starttext
  \setuppapersize[standing]
  Page 1: standing pages from here on. \page
  Page 2. \page
  \setuppapersize[lying]
  Page 3: lying pages from here on. \page
  Page 4. \page
  \adaptpapersize[standing]
  Page 5: adapted to standing \page
  Page 6: automatically back to lying \page
\stoptext