* grep for "TODO" and fix (some of them are easy)
* special variables (done: PWD, PPID, RANDOM)
* follow IFS rules more precisely, including update semantics
+ * builtins mandated by standards we don't support:
+ * [un]alias, command, fc, getopts, newgrp, readonly, times
* export builtin should be special, its arguments are assignments
* and therefore expansion of them should be "one-word" expansion:
* $ export i=`echo 'a b'` # export has one arg: "i=a b"