ppEdge :: Edge -> Doc
ppEdge (EdgeLevels x y)
  = parens (ppLevelSymbol x <+> ppLevelSymbol y)
ppEdge :: Edge -> Doc
ppEdge (EdgeLevels x y)
  = parens (ppLevelSymbol x <+> ppLevelSymbol y)
ppEdge (EdgeLevels x y)
  = parens (ppLevelSymbol x <+> ppLevelSymbol y)