diff --git a/etc/README b/etc/README
index 7547559..7d9f773 100644
--- a/etc/README
+++ b/etc/README
@@ -20,6 +20,8 @@ A Midipix distribution consists of the following:
 * **Debian/-derived Linux**: binutils bzip2 clzip cmake coreutils curl findutils g++ gawk gcc git grep gzip hostname libc6-dev libxml-parser-perl lzma make patch perl procps sed tar util-linux wget xz-utils zip
 * **OpenSUSE Linux**: binutils bzip2 cmake coreutils curl findutils gawk gcc gcc-c++ git grep gzip hostname linux-glibc-devel lzip make patch perl perl-XML-Parser procps sed tar util-linux wget xz zip
 
+> N.B. Busybox is not supported.
+
 > N.B. Some packages (*coreutils*, *grep*, and *tar*, among others) override
 Alpine's BusyBox utilities of the same name, as the latter are either non-
 conformant or defective.
@@ -32,6 +34,46 @@ following command line within the latter:
 ./build.sh -a nt64 -b release -D minipix,zipdist -P -v
 ```
 
+## Non-exhaustive list of build variables
+The following variables are primarily defined in ``midipix.env`` and may be overriden
+on a per-build basis on the command-line after the last argument, if any, e.g.:
+
+```shell
+./build.sh -a nt64 -b release -D minipix,zipdist -P -v PREFIX_ROOT="${HOME}/midipix_tmp"
+```
+
+Furthermore, ``${HOME}/midipix_build.vars``, ``${HOME}/.midipix_build.vars``, and/or
+``../midipix_build.vars`` are sourced during build initialisation and may contain
+additional overrides, particularly ``${DEFAULT_GITROOT_HEAD}``.
+
+| Variable name    | Default value                   | Description                                                                   |
+| ---------------- | ------------------------------- | ----------------------------------------------------------------------------- |
+| ARCH             | nt64                            | Target 32-bit (nt32) or 64-bit (nt64) architecture                            |
+| BUILD            | debug                           | Build w/ debugging (debug) or release compiler flags                          |
+| BUILD_DLCACHEDIR | ${PREFIX_ROOT}/dlcache          | Absolute pathname to package downloads cache root directory                   |
+| BUILD_WORKDIR    | ${PREFIX}/tmp                   | Absolute pathname to temporary package build root directory                   |
+| PREFIX           | ${PREFIX_ROOT}/${ARCH}/${BUILD} | Absolute pathname to architecture- & build type-specific build root directory |
+| PREFIX_CROSS     | ${PREFIX}/${DEFAULT_TARGET}     | Absolute pathname to toolchain root directory                                 |
+| PREFIX_MINGW32   | ${PREFIX}/x86_64-w64-mingw32    | Absolute pathname to MinGW toolchain root directory                           |
+| PREFIX_MINIPIX   | ${PREFIX}/minipix               | Absolute pathname to minipix distribution root directory                      |
+| PREFIX_NATIVE    | ${PREFIX}/native                | Absolute pathname to cross-compiled packages root directory                   |
+| PREFIX_ROOT      | ${HOME}/midipix                 | Absolute pathname to top-level directory                                      |
+| PREFIX_RPM       | ${PREFIX}/rpm                   | Absolute pathname to package RPM archive root directory                       |
+
+## Common tasks
+Rebuild set of packages in isolation, along w/ their dependencies, if any, as needed,
+or forcibly, respectively:
+```shell
+./build.sh [ ... ] -r mc,zsh
+./build.sh [ ... ] -r \*mc,zsh
+./build.sh [ ... ] -r \*\*mc,zsh
+```
+
+Rebuild entire build group:
+```shell
+./build.sh [ ... ] -r ALL native_runtime
+```
+
 ## References
 * ``Sun, 25 Apr 2016 09:04:08 +0000 [1]`` <a href="http://www.musl-libc.org/faq.html" id="r1">musl FAQ</a>  
   
diff --git a/midipix.env b/midipix.env
index 6f87c4b..9b21a4b 100644
--- a/midipix.env
+++ b/midipix.env
@@ -31,15 +31,15 @@ DEFAULT_BUILD_VARS="
 	VERSION";
 
 # Path names
-: ${PREFIX_ROOT:=${HOME}/midipix};
-: ${PREFIX:=${PREFIX_ROOT}/${ARCH}/${BUILD}};
-: ${PREFIX_CROSS:=${PREFIX}/${DEFAULT_TARGET}};
-: ${PREFIX_MINGW32:=${PREFIX}/x86_64-w64-mingw32};
-: ${PREFIX_MINIPIX:=${PREFIX}/minipix};
-: ${PREFIX_NATIVE:=${PREFIX}/native};
-: ${PREFIX_RPM:=${PREFIX}/rpm};
-: ${BUILD_DLCACHEDIR:=${PREFIX_ROOT}/dlcache};
-: ${BUILD_WORKDIR:=${PREFIX}/tmp};
+: ${PREFIX_ROOT:="${HOME}/midipix"};
+: ${PREFIX:="${PREFIX_ROOT}/${ARCH}/${BUILD}"};
+: ${PREFIX_CROSS:="${PREFIX}/${DEFAULT_TARGET}"};
+: ${PREFIX_MINGW32:="${PREFIX}/x86_64-w64-mingw32"};
+: ${PREFIX_MINIPIX:="${PREFIX}/minipix"};
+: ${PREFIX_NATIVE:="${PREFIX}/native"};
+: ${PREFIX_RPM:="${PREFIX}/rpm"};
+: ${BUILD_DLCACHEDIR:="${PREFIX_ROOT}/dlcache"};
+: ${BUILD_WORKDIR:="${PREFIX}/tmp"};
 : ${DEFAULT_CHECK_PATH_VARS:="PREFIX PREFIX_NATIVE PREFIX_CROSS BUILD_DLCACHEDIR BUILD_WORKDIR"};
 : ${DEFAULT_CLEAR_ENV_VARS_EXCEPT:="HOME PATH TERM USER"};
 : ${DEFAULT_CLEAR_PREFIX_PATHS:="