ppEdgeSymbol :: EdgeSymbol -> Doc
ppEdgeSymbol x
  | validEdgeSymbol x = char x
  | otherwise         = error ("ppEdgeSymbol: invalid character: " ++ [x])
ppEdgeSymbol :: EdgeSymbol -> Doc
ppEdgeSymbol x
  | validEdgeSymbol x = char x
  | otherwise         = error ("ppEdgeSymbol: invalid character: " ++ [x])
ppEdgeSymbol x
  | validEdgeSymbol x = char x
  | otherwise         = error ("ppEdgeSymbol: invalid character: " ++ [x])