ppError :: Error -> String
ppError (VarNotInContext name idx context) = "Variable " ++ name
  ++ "[" ++ show idx ++ "] not in context " ++ ppContext context
ppError :: Error -> String
ppError (VarNotInContext name idx context) = "Variable " ++ name
  ++ "[" ++ show idx ++ "] not in context " ++ ppContext context
ppError (VarNotInContext name idx context) = "Variable " ++ name
  ++ "[" ++ show idx ++ "] not in context " ++ ppContext context