Improve documentation regarding Posix utilities

diff --git a/README.markdown b/README.markdown
index 7f5e1df90bd128b3550056f9dc656c7a1a5ff08f..2cefb62ae9079e0fc99979fb61d531a2a6666d6c 100644 (file)
--- a/README.markdown
+++ b/README.markdown
@@ -24,7 +24,7 @@ It aims to do this with zero dependencies beyond the programs already
   installed on your computer†.
 
 † Assuming an operating system with a fairly featureful, and
-  Posix‐compliant, development setup (e·g, macOS).
+  Posix‐compliant, development setup (e·g, Macintosh ≥ version 10.8).
 In fact, on Linux you will probably need to install a few programs:
   `libxml2-utils`, `xsltproc`, `sharutils`, and `pax`.
 
@@ -51,16 +51,33 @@ In Ascii environments, ⛩️📰 书社 should be written `Shushe`, following
 ## Prerequisites
 
 In most cases, ⛩️📰 书社 aims to require only functionality which is
-  present in all Posix‐compliant operating systems.
+  present in all Posix‐compliant (`POSIX.1-2001`) 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.
 
+### `diff`
+
+This is a Posix utility, but ⛩️📰 书社 depends on functionality
+  introduced after `POSIX.1-2001` (the `-u` option, introduced in
+  `POSIX.1-2008`).
+Macintosh systems somewhat interestingly implement this option
+  correctly in legacy mode (`COMMAND_MODE=legacy`) but incorrectly by
+  default (despite claiming `POSIX.1-2008` conformance for this
+  utility).
+[Note this erroneous comment claiming nanosecond & timezone are
+  extensions rather than standardized.][rdar-92753335]
+Despite this, the default Macintosh implementation will still work with
+  ⛩️📰 书社, with the caveat that the timestamp will only include a
+  fractional component when a Posix⹀compliant (e·g, Macintosh legacy or
+  G·N·U) implementation is used.
+
 ### `file`
 
-This is a Posix utility, but ⛩️📰 书社 currently depends on
-  unspecified behaviour.
+This is a Posix utility, but it was considered optional in
+  `POSIX.1-2001` (altho it was made mandatory in `POSIX.1-2008`) and
+  ⛩️📰 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
@@ -88,8 +105,9 @@ To disable it, set `GIT=`.
 
 ### `make`
 
-This is a Posix utility, but ⛩️📰 书社 currently depends on
-  unspecified behaviour.
+This is a Posix utility, but it is considered an optional Software
+  Development utility and ⛩️📰 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
@@ -97,27 +115,30 @@ It is not expected to work in previous versions, or with other
 
 ### `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.
+This is a Posix utility, but it is not included in the Linux Standard
+  Base or installed by default in many distributions.
+⛩️📰 书社 only requires support for the `ustar` format.
 
 ### `uudecode` and `uuencode`
 
-These are Posix utilities, but not included in the Linux Standard Base
-  or installed by default in many distributions.
+These are Posix utilities, but they were considered optional in
+  `POSIX.1-2001` (altho they are made mandatory in `POSIX.1-2008`) and
+  they are 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.
+  provides one implementation.
 
 ### `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`).
+They are a part of `libxml2`, but may need to be installed separately
+  on some platforms (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.
+It is a part of `libxslt`, but may need to be installed separately on
+  some platforms.
 
 ## Basic Usage
 
@@ -702,3 +723,4 @@ Most source files are licensed under the terms of the <cite>Mozilla
 
 [REUSE]: <https://reuse.software/spec/>
 [draft-phillips-record-jar-01]: <https://datatracker.ietf.org/doc/html/draft-phillips-record-jar-01>
+[rdar-92753335]: <https://github.com/apple-oss-distributions/patch_cmds/blob/5084833f90df1b0e0924ea56f94c0199b3b8bbc6/diff/diffreg.c#L1800-L1808>