Front End

  [Usage]         oils-usage   ysh-usage             shell-flags
                  config       startup               line-editing          
                  exit-codes
  [Lexing]        comment #    line-continuation \   ascii-whitespace [ \t\r\n]
  [Lexing]        doc-comment ###                    multiline-command ...
  [Tools]         cat-em                             syntax-tree
