Options parsing didn't support "default" (shown by the 'O' command)
or "Default symbols" (menu entry for choosing a symbol set via the
'O' command. Symbol handling is somewhat confusing, but this seems to
do the trick. They can't be truncated, but they're case-insensitive,
and "Default" and "symbols" can be separated by dash or underscore as
well as space, or run-together with no separator.
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