ppOutputSymbol :: OutputSymbol -> Doc
ppOutputSymbol x
  | validOutputSymbol x = char x
  | otherwise           = error ("ppOutputSymbol: invalid character: " ++ [x])
ppOutputSymbol :: OutputSymbol -> Doc
ppOutputSymbol x
  | validOutputSymbol x = char x
  | otherwise           = error ("ppOutputSymbol: invalid character: " ++ [x])
ppOutputSymbol x
  | validOutputSymbol x = char x
  | otherwise           = error ("ppOutputSymbol: invalid character: " ++ [x])