diff --git a/provenience.cabal b/provenience.cabal
--- a/provenience.cabal
+++ b/provenience.cabal
@@ -1,5 +1,5 @@
 name:                provenience
-version:             0.1.0.2
+version:             0.1.1.0
 synopsis:            Computations that automatically track data dependencies
 description:         see README.md
 license:             GPL-3
diff --git a/src/Control/Provenience.hs b/src/Control/Provenience.hs
--- a/src/Control/Provenience.hs
+++ b/src/Control/Provenience.hs
@@ -88,6 +88,7 @@
     ,ProvenienceT(..)
     ,Provenience
     ,(<%>)
+    ,(<%?>)
     ,(<%%>)
     ,runProvenienceT
     ,execProvenienceT
@@ -396,7 +397,7 @@
 named :: Variable a -> String -> StoreUpdate
 v `named` name = modify' (\vs -> changeLabel (identifier v) (\desc -> desc {shortname = Just name}) vs)
 
-infixl 4 <%>, <%%>
+infixl 4 <%>, <%?>, <%%>
 -- | 'Applicative'-style application operator. 
 -- Replaces the function 'Variable' with its (partial) application 
 -- and creates an edge from the argument to the result 'Variable'
@@ -429,6 +430,13 @@
 -- @
 (<%>) :: Monad m => ProvenienceT alt m (a -> b) -> Variable a -> ProvenienceT alt m b
 pf <%> x = ((fmap.fmap) (pure.) pf) <%%> x
+
+-- | Conditional use of a 'Variable'. 
+-- The @'Left' a@ could be a default parameter to the function 
+-- which is not worth registering a 'Variable' for. 
+(<%?>) :: Monad m => ProvenienceT alt m (a -> b) -> Either a (Variable a) -> ProvenienceT alt m b
+pf <%?> (Right px) = pf <%> px
+pf <%?> (Left   x) = (fmap.fmap) ($x) pf
 
 -- | Like above but permit side-effects in the base monad. 
 (<%%>) :: Monad m => ProvenienceT alt m (a -> m b) -> Variable a -> ProvenienceT alt m b