diff --git a/relational-query.cabal b/relational-query.cabal
--- a/relational-query.cabal
+++ b/relational-query.cabal
@@ -1,5 +1,5 @@
 name:                relational-query
-version:             0.0.1.6
+version:             0.0.1.7
 synopsis:            Typeful, Modular, Relational, algebraic query engine
 description:         This package contiains typeful relation structure and
                      relational-algebraic query building DSL which can
diff --git a/src/Database/Relational/Query/Pi/Unsafe.hs b/src/Database/Relational/Query/Pi/Unsafe.hs
--- a/src/Database/Relational/Query/Pi/Unsafe.hs
+++ b/src/Database/Relational/Query/Pi/Unsafe.hs
@@ -100,11 +100,11 @@
 (<.>) :: Pi a b -> Pi b c -> Pi a c
 (<.>) = unsafePiAppend id
 
--- | Compose projection path.
+-- | Compose projection path. 'Maybe' phantom functor is 'map'-ed.
 (<?.>) :: Pi a (Maybe b) -> Pi b c -> Pi a (Maybe c)
 (<?.>) = unsafePiAppend maybeWidth
 
--- | Compose projection path.
+-- | Compose projection path. 'Maybe' phantom functors are 'join'-ed like '>=>'.
 (<?.?>) :: Pi a (Maybe b) -> Pi b (Maybe c) -> Pi a (Maybe c)
 (<?.?>) = unsafePiAppend id
 
diff --git a/src/Database/Relational/Query/ProjectableExtended.hs b/src/Database/Relational/Query/ProjectableExtended.hs
--- a/src/Database/Relational/Query/ProjectableExtended.hs
+++ b/src/Database/Relational/Query/ProjectableExtended.hs
@@ -156,7 +156,7 @@
 
 -- | Get narrower projection along with projection path
 --   and project into result projection type.
---   'Maybe' phantom type is propagated.
+--   'Maybe' phantom functor is 'map'-ed.
 (?!) :: Projectable Projection p
      => Projection c (Maybe a) -- ^ Source 'Projection'. 'Maybe' type
      -> Pi a b       -- ^ Projection path
@@ -165,7 +165,7 @@
 
 -- | Get narrower projection along with projection path
 --   and project into result projection type.
---   'Maybe' phantom type is propagated. Projection path leaf is 'Maybe' case.
+--   Source record 'Maybe' phantom functor and projection path leaf 'Maybe' functor are 'join'-ed.
 (?!?) :: Projectable Projection p
       => Projection c (Maybe a)   -- ^ Source 'Projection'. 'Maybe' phantom type
       -> Pi a (Maybe b) -- ^ Projection path. 'Maybe' type leaf