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  [Arithmetic]    arith-context Where legacy arithmetic is allowed
                  num-literals  0xFF  0755  etc.
                  math          1 + 2*3
                  arith-logical !a && b
                  bitwise       ~a ^ b
                  arith-assign  a *= 2
  [Boolean]       dbracket      [[ vs. the test builtin
                  bool-expr       [[ ! $x && $y || $z ]]
                                test ! $x -a $y -o $z
                  bool-infix    [[ $a -nt $b ]]  [[ $x == $y ]]
                  bool-path     [[ -d /etc ]]
                  bool-str      [[ -z '' ]]
                  bool-other    [[ -o errexit ]]
  [Patterns]      glob          *.py
                  extglob       @(*.py|*.sh)
                  regex         [[ foo =~ [a-z]+ ]]
  [Brace Expand]  braces        {alice,bob}@example.com
  [History]       histsub       !$  !!  !n
