1 files changed, 2 insertions, 1 deletions

diff --git a/rpn.hs b/rpn.hs
index 5c75fcf..dbb0f1f 100644..100755
--- a/rpn.hs
+++ b/rpn.hs
@@ -67,7 +67,7 @@ clarifySymbol s = do
 
 isKnownSymbol :: String -> Bool
 isKnownSymbol s = or [
-  s `elem` ["+", "-", "*", "/", "±", "e"],
+  s `elem` ["+", "-", "*", "/", "±", "pm", "e"],
   isJust $ (maybeRead s :: Maybe Double)
                      ]
 
@@ -85,6 +85,7 @@ interpreteSymbol "-" = Sub
 interpreteSymbol "*" = Mult
 interpreteSymbol "/" = Div
 interpreteSymbol "±" = Unc
+interpreteSymbol "+-" = Unc
 interpreteSymbol "e" = Exp
 interpreteSymbol x = Number (read x) 0