explicit keyword not INIT

Author: mcabbott

src/functor.jl

@@ -43,10 +43,10 @@ function _default_walk(f, x)
   re(map(f, func))
 end
 
-const INIT = Base._InitialValue()  # sentinel value for keyword not supplied
+@gensym nokeyword
 
-function fmap(f, x; exclude = isleaf, walk = _default_walk, cache = IdDict(), prune = INIT)
-  haskey(cache, x) && return prune === INIT ? cache[x] : prune
+function fmap(f, x; exclude = isleaf, walk = _default_walk, cache = IdDict(), prune = nokeyword)
+  haskey(cache, x) && return prune === nokeyword ? cache[x] : prune
   cache[x] = exclude(x) ? f(x) : walk(x -> fmap(f, x; exclude, walk, cache, prune), x)
 end