+ * The build_command field of the project config file is used to invoke
+ * the relevant build command. This command tells SCons where to find
+ * the rules.
+ *
+ * Our chicken-and-egg dilemma is this: we want to use the version of
+ * SCons under development in an Aegis change to build itself. But the
+ * pieces of SCons are likely only partly in this change, and partly in
+ * baselines.
+ *
+ * Python only imports things on a module-by-module basis--which is to
+ * say, once it finds __init__.py in a given directory, it assumes that
+ * all other files in that module are in the same directory. But that's
+ * not the way Aegis works, because if a file hasn't changed on the
+ * branch, it will only be in its parent's baseline directory.
+ *
+ * Aegis' mechanism for working around this sort of problem is to make
+ * symlinks to the proper baseline versions of each file, which makes
+ * it look like everything is in the local tree. That's unattractive,
+ * though, because we really want to eat our own dog food and use the
+ * SCons -Y options to pull things from the baseline repositories.
+ *
+ * So our solution (suggested by Anthony Roach) is a bootstrap.py script
+ * that does some Aegis-like searching through the baseline directories
+ * and makes a bootstrap copy of the version of SCons under development
+ * that we can use for building. After it makes this copy of SCons, it
+ * executes it with the same command-line arguments we supplied (and
+ * setting $SCONS_LIB_DIR to the right directory) so we can use it
+ * here with command-line options as if it were SCons itself. (Note,
+ * however, that bootstrap.py only understands the specific command-line
+ * options already in use here, so if you change the call below to add
+ * some other SCons options, you may have to modify bootstrap.py to
+ * recognize them.
+ *
+ * The ${Source bootstrap.py} substitution finds bootstrap.py wherever
+ * it may be in the Aegis baselines.
+ *
+ * The long -Y${SUBSTitute...} substitution takes the Aegis baseline
+ * search path and turns it into the right -Y command-line options for
+ * SCons.
+ *
+ * The rest of the substitutions (${DEVeloper}, etc.) should be obvious.
+ *
+ * Look in aesub(5) for more information about command substitutions.