+## Prerequisites
+
+In most cases, ⛩️📰 书社 aims to require only functionality which is
+ present in all Posix‐compliant operating systems.
+There are a few exceptions.
+Details on particular programs are given below; if a program is not
+ listed, it is assumed that any Posix‐compliant implementation will
+ work.
+
+### `date`
+
+This is a Posix utility, but ⛩️📰 书社 currently depends on
+ unspecified behaviour.
+When the G·N·U version of `stat` is being used, then the G·N·U version
+ of `date` is also expected.
+
+### `file`
+
+This is a Posix utility, but ⛩️📰 书社 currently depends on
+ unspecified behaviour.
+It requires support for the following additional options :—
+
+- **`-C`**, when supplied with `-m`, must be useable to compile a
+ `.mgc` magicfile for use with future invocations of `file`.
+
+- **`--files-from`** must be useable to provide a file that `file`
+ should read file·names from, and `-` must be useable in this
+ context to specify the standard input.
+
+- **`--mime-type`** must cause `file` to print the internet media type
+ of the file with no charset parameter.
+
+- **`--separator`** must be useable to set the separator that `file`
+ uses to separate file names from types.
+
+These options are implemented by the
+ [Fine Free File Command](https://darwinsys.com/file/), which is used
+ by most operating systems.
+
+### `git`
+
+This is not a Posix utility.
+Usage of `git` is optional, but recommended (and activated by default).
+To disable it, set `GIT=`.
+
+### `make`
+
+This is a Posix utility, but ⛩️📰 书社 currently depends on
+ unspecified behaviour.
+⛩️📰 书社 requires specifically the G·N·U version of `make`, and
+ depends on functionality present in version 3.81 or later.
+It is not expected to work in previous versions, or with other
+ implementations of Make.
+
+### `pax`
+
+This is a Posix utility, but not included in the Linux Standard Base or
+ installed by default in many distributions.
+Only `ustar` format support is required.
+
+### `stat`
+
+This is not a Posix utility, and nor is it particularly portable.
+To get around incompatibilities, ⛩️📰 书社 attempts to recognize G·N·U
+ `stat` by searching for the string `GNU` when invoked with the
+ `--version` option, and falls back to B·S·D behaviour otherwise.
+
+### `uudecode` and `uuencode`
+
+These are Posix utilities, but not included in the Linux Standard Base
+ or installed by default in many distributions.
+The G·N·U [Sharutils](https://www.gnu.org/software/sharutils/) package
+ can be installed to access them.
+
+### `xargs`
+
+This is a Posix utility, but ⛩️📰 书社 currently depends on
+ unspecified behaviour.
+It requires support for the `-0` flag, which must disable the special
+ quote and whitespace handling of `xargs` in favour of null‐terminated
+ strings.
+
+### `xmlcatalog` and `xmllint`
+
+These are not a Posix utilities.
+They is a part of `libxml2`, but may need to be installed separately
+ (e·g by the name `libxml2-utils`).
+
+### `xsltproc`
+
+This is not a Posix utility.
+It is a part of `libxslt`, but may need to be installed separately.
+