Special Variables

  [YSH Vars]      ARGV   X ENV  X _ESCAPE
                  _this_dir
  [YSH Status]    _status   _error
                  _pipeline_status   _process_sub_status
  [YSH Tracing]   SHX_indent   SHX_punct   SHX_pid_str
  [YSH read]      _reply
  [History]       YSH_HISTFILE
  [Oils VM]       OILS_VERSION
                  OILS_GC_THRESHOLD   OILS_GC_ON_EXIT
                  OILS_GC_STATS   OILS_GC_STATS_FD
X [Wok]           _filename   _line
X [Builtin Sub]   _buffer
