story: generators: add check for invalid case

pkg/arvo/gen/hood/story-remove.hoon

@@ -15,7 +15,9 @@
 ::      leaning towards no, and potentially even making the case non-optional
 =/  our                p.bec
 =?  cas  =(*case cas)  r.bec  :: use case from beak if cas not provided
-:: TODO case existence check
+?:  !.^(? %cs /(scot %cs /(scot %p our)/[desk]/(scot cas)/case))
+  ~&  >>  "Error: invalid case {<cas>} provided"
+  helm-pass+[%d %noop ~]
 =/  tak
   ?:  ?=([%tako tako:clay] cas)
     p.cas

pkg/arvo/gen/hood/story-set.hoon

@@ -13,7 +13,9 @@
 ?:  !(~(has in .^((set ^desk) %cd /(scot %p our)/$/(scot %da now))) desk)
   ~&  >>  "Error: desk {<desk>} does not exist."
   helm-pass+[%d %noop ~]
-::  TODO case existence check
+?:  !.^(? %cs /(scot %cs /(scot %p our)/[desk]/(scot cas)/case))
+  ~&  >>  "Error: invalid case {<cas>} provided"
+  helm-pass+[%d %noop ~]
 =/  tak
   ?:  ?=([%tako tako:clay] cas)
     p.cas

pkg/base-dev/lib/story.hoon

@@ -11,7 +11,6 @@
   ::  0 \ b = 0  :: anything in 0 but not in b is by definition 0
   ::
   ?:  =(~ a)  ~
-  ~&  a
   ::  uno := (a-b) + (merged items in both a and b) + (b-a)
   ::  ret := (a-b) + (merged items in both a and b)  
   ::  ret = (~(int by a) uno)  :: preserve only the entries whose keys are in a