diff --git a/demo/ClipPic.hs b/demo/ClipPic.hs
--- a/demo/ClipPic.hs
+++ b/demo/ClipPic.hs
@@ -20,7 +20,8 @@
 pic1 :: Picture
 pic1 = frame [ body ]
   where
-    body = clip dog_house $ primGroup [ red_circle, green_circle, blue_circle ]
+    body = clipPrim dog_house $ 
+             primGroup [ red_circle, green_circle, blue_circle ]
 
 red_circle :: Primitive 
 red_circle = fillEllipse red 60 60      $ P2 (-20) 0
diff --git a/demo/EllipsePic.hs b/demo/EllipsePic.hs
--- a/demo/EllipsePic.hs
+++ b/demo/EllipsePic.hs
@@ -34,5 +34,5 @@
 
 ellipse03 :: DPoint2 -> Primitive
 ellipse03 pt = cstroke red default_stroke_attr $ 
-    curvedPrimPath $ rbezierEllipse 20 30 (negate $ d2r (10::Double)) pt
+    curvedPrimPath $ rbezierEllipse 20 30 (negate $ d2r 10) pt
 
diff --git a/demo/Hyperlink.hs b/demo/Hyperlink.hs
--- a/demo/Hyperlink.hs
+++ b/demo/Hyperlink.hs
@@ -11,8 +11,8 @@
 main :: IO ()
 main = do 
     createDirectoryIfMissing True "./out/"
-    writeEPS "./out/svg_link01.eps"  link_pic
-    writeSVG "./out/svg_link01.svg"  link_pic
+    writeEPS "./out/hyperlink01.eps"  link_pic
+    writeSVG "./out/hyperlink01.svg"  link_pic
 
 
 link_pic :: Picture
diff --git a/demo/TransformEllipse.hs b/demo/TransformEllipse.hs
--- a/demo/TransformEllipse.hs
+++ b/demo/TransformEllipse.hs
@@ -42,7 +42,7 @@
      ell  = mkRedEllipse (rotate ang) 20 10 pt
      cb   = rotate ang $ crossbar 20 10 pt
      pt   = P2 70 10
-     ang  = d2r (30::Double)
+     ang  = d2r 30
 
 pic3 :: Picture
 pic3 = cb `picOver` ell `picOver` xy_frame "rotateAbout (60,0) 30deg"
@@ -51,7 +51,7 @@
      cb   = rotateAbout ang pto $ crossbar 20 10 pt
      pt   = P2 70 10
      pto  = P2 60 0 `asTypeOf` dpt
-     ang  = d2r (30::Double)
+     ang  = d2r 30
 
 
 pic4 :: Picture
diff --git a/demo/TransformPath.hs b/demo/TransformPath.hs
--- a/demo/TransformPath.hs
+++ b/demo/TransformPath.hs
@@ -40,7 +40,7 @@
      pth  = mkBlackPath (rotate ang) pt
      ch   = rotate ang $ zcrosshair pt
      pt   = P2 70 10
-     ang  = d2r (30::Double)
+     ang  = d2r 30
 
 
 pic3 :: Picture
@@ -50,7 +50,7 @@
      ch   = rotateAbout ang pto $ zcrosshair pt
      pt   = P2 70 10
      pto  = P2 60 0 `asTypeOf` dpt
-     ang  = d2r (30::Double)
+     ang  = d2r 30
     
 
 pic4 :: Picture
diff --git a/demo/TransformTextlabel.hs b/demo/TransformTextlabel.hs
--- a/demo/TransformTextlabel.hs
+++ b/demo/TransformTextlabel.hs
@@ -40,7 +40,7 @@
     txt  = mkBlackTextlabel (rotate ang) pt
     ch   = rotate ang $ zcrosshair pt
     pt   = P2 70 10
-    ang  = d2r (30::Double)
+    ang  = d2r 30
 
 
 pic3 :: Picture
@@ -50,7 +50,7 @@
     ch   = rotateAbout ang pto $ zcrosshair pt
     pt   = P2 70 10
     pto  = P2 60 0 `asTypeOf` dpt
-    ang  = d2r (30::Double)
+    ang  = d2r 30
 
 
 pic4 :: Picture
diff --git a/src/Wumpus/Core/Geometry.hs b/src/Wumpus/Core/Geometry.hs
--- a/src/Wumpus/Core/Geometry.hs
+++ b/src/Wumpus/Core/Geometry.hs
@@ -60,10 +60,13 @@
   , vvec
   , avec
   , pvec
+  , orthoVec
   , vreverse
   , vdirection
   , vlength
   , vangle
+  , vsum
+  , vdiff
 
   -- * Point operations
   , zeroPt
@@ -564,6 +567,15 @@
 pvec = flip (.-.)
 
 
+
+-- | Build a vector form its parallel and perpendicular components
+-- and inclination.
+--
+orthoVec :: Floating u => u -> u -> Radian -> Vec2 u 
+orthoVec pall perp ang = avec ang pall ^+^ avec (ang + half_pi) perp
+  where
+    half_pi = 0.5 * pi
+
 -- | 'vreverse' : @ vec -> Vec2 @
 --
 -- Reverse a vector.
@@ -598,6 +610,26 @@
 vangle u v = realToFrac $ acos $ (u <.> v) / (magnitude u * magnitude v)
 
 
+-- | Sum a list of Vectors.
+--
+vsum :: Num u => [Vec2 u] -> Vec2 u
+vsum [] = V2 0 0
+vsum (v:vs) = go v vs
+  where
+    go a []     = a
+    go a (b:bs) = go (a ^+^ b) bs
+
+
+-- | Find the /difference/ between two vectors - i.e. flipped
+-- vector subtraction:
+--
+-- > vdiff = flip (^-^)
+-- 
+vdiff :: Num u => Vec2 u -> Vec2 u -> Vec2 u
+vdiff  = flip (^-^)
+
+
+
 --------------------------------------------------------------------------------
 -- Points
 
@@ -849,12 +881,18 @@
 
 -- | Degrees to radians.
 --
-d2r :: (Floating a, Real a) => a -> Radian
+-- Degree type fixed to @Double@, compose @d2r@ with @realToFrac@ 
+-- for @Float@ etc.
+--
+d2r :: Double -> Radian
 d2r = Radian . realToFrac . (*) (pi/180)
 
 -- | Radians to degrees.
 --
-r2d :: (Floating a, Real a) => Radian -> a
+-- Degree type fixed to @Double@, compose @r2d@ with @realToFrac@ 
+-- for @Float@ etc.
+--
+r2d :: Radian -> Double
 r2d = (*) (180/pi) . fromRadian
 
 
diff --git a/src/Wumpus/Core/OutputPostScript.hs b/src/Wumpus/Core/OutputPostScript.hs
--- a/src/Wumpus/Core/OutputPostScript.hs
+++ b/src/Wumpus/Core/OutputPostScript.hs
@@ -431,7 +431,7 @@
 -- encoded in the matrix, hence the @ 0 0 moveto @.
 --
 primLabel :: LabelProps -> PrimLabel -> PsMonad Doc
-primLabel (LabelProps rgb attrs) (PrimLabel body ctm) = bracketPrimCTM ctm mf
+primLabel (LabelProps rgb attrs) (PrimLabel body _ ctm) = bracketPrimCTM ctm mf
   where
     ev    = font_enc_vector $ font_face attrs    
     mf pt = (\rgbd fontd -> vcat [ rgbd, fontd, labelBody ev pt body ]) 
diff --git a/src/Wumpus/Core/OutputSVG.hs b/src/Wumpus/Core/OutputSVG.hs
--- a/src/Wumpus/Core/OutputSVG.hs
+++ b/src/Wumpus/Core/OutputSVG.hs
@@ -240,6 +240,7 @@
 svgAnnoPrim (ALink hypl)    d = drawXLink hypl d
 svgAnnoPrim (GAnno xs)      d = drawGProps xs d
 svgAnnoPrim (SvgAG hypl xs) d = drawXLink hypl $ drawGProps xs d 
+svgAnnoPrim (SvgId ss)      d = drawGProps [SvgAttr "id" ss] d 
 
 
 drawXLink :: XLink -> Doc -> Doc
@@ -346,13 +347,13 @@
 --
 
 primLabel :: LabelProps -> PrimLabel -> SvgMonad Doc
-primLabel (LabelProps rgb attrs) (PrimLabel body ctm) = 
-    (\fa ca -> elem_text (fa <+> ca) (makeTspan rgb dtext))
+primLabel (LabelProps rgb attrs) (PrimLabel body opt_id ctm) = 
+    (\fa ca -> elem_text (id_f $ fa <+> ca) (makeTspan rgb dtext))
       <$> deltaFontAttrs attrs <*> bracketTextCTM ctm coordf
-                               
   where
     coordf = \p0 -> pure $ labelBodyCoords body p0
     dtext  = labelBodyText body
+    id_f   = maybe id (\xid -> (svgAttr "id" (text xid) <+>)) $ opt_id
 
 labelBodyCoords :: LabelBody -> DPoint2 -> Doc
 labelBodyCoords (StdLayout _)  pt = makeXY pt
diff --git a/src/Wumpus/Core/PageTranslation.hs b/src/Wumpus/Core/PageTranslation.hs
--- a/src/Wumpus/Core/PageTranslation.hs
+++ b/src/Wumpus/Core/PageTranslation.hs
@@ -66,8 +66,8 @@
 
 
 trivLabel :: PrimLabel -> PrimLabel
-trivLabel (PrimLabel txt ctm) = 
-    PrimLabel txt (trivPrimCTM ctm)
+trivLabel (PrimLabel txt opt_id ctm) = 
+    PrimLabel txt opt_id (trivPrimCTM ctm)
 
 trivEllipse :: PrimEllipse -> PrimEllipse
 trivEllipse (PrimEllipse hw hh ctm) = PrimEllipse hw hh (trivPrimCTM ctm)
diff --git a/src/Wumpus/Core/Picture.hs b/src/Wumpus/Core/Picture.hs
--- a/src/Wumpus/Core/Picture.hs
+++ b/src/Wumpus/Core/Picture.hs
@@ -50,6 +50,7 @@
   , curvedPrimPath
   , xlinkhref
   , xlinkPrim
+  , xidPrim
   , svgattr
   , annotateGroup
   , annotateXLink
@@ -68,7 +69,7 @@
   , zfill
 
   , fillStroke
-  , clip
+  , clipPrim
 
   , textlabel
   , rtextlabel
@@ -333,14 +334,28 @@
 xlinkhref = XLink
 
 
--- | Create a hyperlinked Primitive.
+-- | Hyperlinked Primitive.
 --
+-- This encloses the Primitive in an addition @\<g\>@ element.
+--
 -- Note - hyperlinks are ignored in the PostScript output.
 --
 xlinkPrim :: XLink -> Primitive -> Primitive
 xlinkPrim hypl p = PSVG (ALink hypl) p  
 
 
+-- | Annotate a Primitive with an @id@. 
+-- 
+-- For text labels this annotates the label directly, for other
+-- primitives the annotation adds extra nesting with a @\<g\>@ 
+-- element.
+--
+-- Note - ids are ignored in the PostScript output.
+--
+xidPrim :: String -> Primitive -> Primitive
+xidPrim = pushXIdAnno
+
+
 -- | Create an attribute for SVG output.
 --
 -- Attributes are expected to be /non-graphical/ e.g. @onclick@ 
@@ -488,12 +503,12 @@
 --------------------------------------------------------------------------------
 -- Clipping 
 
--- | 'clip' : @ path * primitive -> Primitive @
+-- | 'clipPrim' : @ path * primitive -> Primitive @
 -- 
 -- Clip a primitive to be inside the supplied path.
 --
-clip :: PrimPath -> Primitive -> Primitive
-clip cp p = PClip cp p
+clipPrim :: PrimPath -> Primitive -> Primitive
+clipPrim cp p = PClip cp p
 
 --------------------------------------------------------------------------------
 -- Labels to primitive
@@ -557,7 +572,7 @@
               -> Primitive
 rescapedlabel rgb attr txt theta (P2 dx dy) = PLabel (LabelProps rgb attr) lbl 
   where
-    lbl = PrimLabel (StdLayout txt) (makeThetaCTM dx dy theta)
+    lbl = PrimLabel (StdLayout txt) Nothing (makeThetaCTM dx dy theta)
 
 
 -- | 'zescapedlabel' : @ escaped_text * baseline_left -> Primitive @
@@ -605,7 +620,7 @@
 hkernlabel :: RGBi -> FontAttr -> [KerningChar] -> DPoint2 -> Primitive
 hkernlabel rgb attr xs (P2 x y) = PLabel (LabelProps rgb attr) lbl 
   where
-    lbl = PrimLabel (KernTextH xs) (makeTranslCTM x y)
+    lbl = PrimLabel (KernTextH xs) Nothing (makeTranslCTM x y)
 
 
 
@@ -645,7 +660,7 @@
 vkernlabel :: RGBi -> FontAttr -> [KerningChar] -> DPoint2 -> Primitive
 vkernlabel rgb attr xs (P2 x y) = PLabel (LabelProps rgb attr) lbl 
   where
-    lbl = PrimLabel (KernTextV xs) (makeTranslCTM x y)
+    lbl = PrimLabel (KernTextV xs) Nothing (makeTranslCTM x y)
 
 
 
diff --git a/src/Wumpus/Core/PictureInternal.hs b/src/Wumpus/Core/PictureInternal.hs
--- a/src/Wumpus/Core/PictureInternal.hs
+++ b/src/Wumpus/Core/PictureInternal.hs
@@ -51,6 +51,8 @@
   , isEmptyPath
   , isEmptyLabel
 
+  , pushXIdAnno
+
   ) where
 
 import Wumpus.Core.AffineTrans
@@ -175,15 +177,18 @@
 
 -- | SVG annotations - annotations can be: 
 -- 
--- A hyperlink inside @<a ...> ... </a>@ .
+-- * A hyperlink inside @<a ...> ... </a>@ .
 --
--- A group - @<g ...> ... </g>@
+-- * A group - @<g ...> ... </g>@
 --
--- A group inside a hyperlink.
+-- * A group of annotations inside a hyperlink.
 --
+-- * An @id@.
+--
 data SvgAnno = ALink XLink
              | GAnno [SvgAttr]
              | SvgAG XLink [SvgAttr]
+             | SvgId String
    deriving (Eq,Show)
 
 
@@ -284,6 +289,7 @@
 --
 data PrimLabel = PrimLabel 
       { label_body      :: LabelBody
+      , label_opt_id    :: Maybe String
       , label_ctm       :: PrimCTM
       }
   deriving (Eq,Show)
@@ -420,8 +426,9 @@
   format (RelLineTo pt)         = text "rel_line_to  " <> format pt
 
 instance Format PrimLabel where
-  format (PrimLabel s ctm) = 
+  format (PrimLabel s opt_id ctm) = 
      vcat [ dquotes (format s)
+          , maybe (char '_') text $ opt_id 
           , text "ctm="           <> format ctm
           ]
 
@@ -508,7 +515,7 @@
 
 
 labelBoundary :: FontAttr -> PrimLabel -> BoundingBox Double
-labelBoundary attr (PrimLabel body ctm) = 
+labelBoundary attr (PrimLabel body _ ctm) = 
     retraceBoundary (m33 *#) untraf_bbox
   where
     m33         = matrixRepCTM ctm
@@ -720,26 +727,27 @@
 -- label.
 --
 rotateLabel :: Radian -> PrimLabel -> PrimLabel
-rotateLabel ang (PrimLabel txt ctm) = PrimLabel txt (rotateCTM ang ctm)
+rotateLabel ang (PrimLabel txt opt_id ctm) = 
+    PrimLabel txt opt_id (rotateCTM ang ctm)
 
 
 -- /rotateAbout/ the start-point, /rotate/ the the CTM.
 --
 rotateAboutLabel :: Radian -> DPoint2 -> PrimLabel -> PrimLabel
-rotateAboutLabel ang (P2 x y) (PrimLabel txt ctm) = 
-    PrimLabel txt (rotateAboutCTM ang (P2 x y) ctm)
+rotateAboutLabel ang (P2 x y) (PrimLabel txt opt_id ctm) = 
+    PrimLabel txt opt_id (rotateAboutCTM ang (P2 x y) ctm)
 
 
 scaleLabel :: Double -> Double -> PrimLabel -> PrimLabel
-scaleLabel sx sy (PrimLabel txt ctm) = 
-    PrimLabel txt (scaleCTM sx sy ctm)
+scaleLabel sx sy (PrimLabel txt opt_id ctm) = 
+    PrimLabel txt opt_id (scaleCTM sx sy ctm)
 
 
 -- Change the bottom-left corner.
 --
 translateLabel :: Double -> Double -> PrimLabel -> PrimLabel
-translateLabel dx dy (PrimLabel txt ctm) = 
-    PrimLabel txt (translateCTM dx dy ctm)
+translateLabel dx dy (PrimLabel txt opt_id ctm) = 
+    PrimLabel txt opt_id (translateCTM dx dy ctm)
 
 --------------------------------------------------------------------------------
 -- Ellipse
@@ -852,9 +860,21 @@
 -- | Is the label empty - if so we might want to avoid printing it.
 --
 isEmptyLabel :: PrimLabel -> Bool
-isEmptyLabel (PrimLabel txt _) = body txt
+isEmptyLabel (PrimLabel txt _ _) = body txt
    where
      body (StdLayout esc) = destrEscapedText null esc
      body (KernTextH xs)  = null xs
      body (KernTextV xs)  = null xs
 
+
+-- | Annotate a Primitive with an @id@ for SVG.
+--
+-- Note - for @PLabel@ this /pushes/ the id /inside/ the 
+-- constructor, for other elements the the id adds an extra layer 
+-- of nesting via the SVG group \<g\> tag.
+-- 
+pushXIdAnno :: String -> Primitive -> Primitive
+pushXIdAnno ss (PLabel props (PrimLabel txt _ ctm )) = 
+    PLabel props $ PrimLabel txt (Just ss) ctm
+ 
+pushXIdAnno ss prim                                  = PSVG (SvgId ss) prim
diff --git a/src/Wumpus/Core/VersionNumber.hs b/src/Wumpus/Core/VersionNumber.hs
--- a/src/Wumpus/Core/VersionNumber.hs
+++ b/src/Wumpus/Core/VersionNumber.hs
@@ -22,7 +22,7 @@
 
 -- | Version number.
 --
--- > (0,51,0)
+-- > (0,52,0)
 --
 wumpus_core_version :: (Int,Int,Int)
-wumpus_core_version = (0,51,0)
+wumpus_core_version = (0,52,0)
diff --git a/src/Wumpus/Core/WumpusTypes.hs b/src/Wumpus/Core/WumpusTypes.hs
--- a/src/Wumpus/Core/WumpusTypes.hs
+++ b/src/Wumpus/Core/WumpusTypes.hs
@@ -37,6 +37,7 @@
   , FontCtx
   , Primitive
   , XLink
+  , SvgAttr
   , PrimPath
   , PrimPathSegment
   , AbsPathSegment
diff --git a/wumpus-core.cabal b/wumpus-core.cabal
--- a/wumpus-core.cabal
+++ b/wumpus-core.cabal
@@ -1,5 +1,5 @@
 name:             wumpus-core
-version:          0.51.0
+version:          0.52.0
 license:          BSD3
 license-file:     LICENSE
 copyright:        Stephen Tetley <stephen.tetley@gmail.com>
@@ -47,6 +47,21 @@
   .  
   .
   Changelog:
+  . 
+  v0.51.0 to v0.52.0:
+  .
+  * Added limited support for adding SVG ids to text and 
+    Primitives.
+  .
+  * Fixed types of @d2r@ and @r2d@ to Double for degrees, rather 
+    than a parametric (Real a, Floating a) type. Although this is
+    less general, it removes the burden of type annotating the 
+    common case.
+  .
+  * Added the vector functions @orthoVec@, @vsum@ and @vdiff@ to 
+    @Core.Geometry@.
+  .
+  * Re-named @clip@ to @clipPrimitive@
   .
   v0.50.0 to v0.51.0:
   .