Merge pull request #6898 from urbit/pkova/grab

clay: do not return false conversion gate from build-cast

pkg/arvo/app/herm.hoon

@@ -5,7 +5,6 @@
 ::  keep relevant mark conversions in cache for performance
 ::
 /$  blit-to-json  %blit  %json
-/$  json-to-blit  %json  %blit
 /$  json-to-task  %json  %herm-task
 ::
 =,  jael

pkg/arvo/sys/vane/clay.hoon

@@ -729,11 +729,7 @@
       ::
       %-  (trace 1 |.("make cast {<a>} -> {<b>}"))
       =^  old=vase  nub  (build-fit %mar a)
-      ?:  =/  ram  (mule |.((slap old !,(*hoon grow))))
-          ?:  ?=(%| -.ram)  %.n
-          =/  lab  (mule |.((slob b p.p.ram)))
-          ?:  ?=(%| -.lab)  %.n
-          p.lab
+      ?:  (has-arm %grow b old)
         ::  +grow core has .b arm; use that
         ::
         %+  gain-leak  cast+a^b
@@ -749,8 +745,9 @@
       ::  try direct +grab
       ::
       =^  new=vase  nub  (build-fit %mar b)
+      =/  arm=?  (has-arm %grab a new)
       =/  rab  (mule |.((slap new tsgl/[limb/a limb/%grab])))
-      ?:  &(?=(%& -.rab) ?=(^ q.p.rab))
+      ?:  &(arm ?=(%& -.rab) ?=(^ q.p.rab))
         %+  gain-leak  cast+a^b
         |=  nob=state
         %-  (trace 4 |.("{<a>} -> {<b>}: +{(trip a)}:grab:{(trip b)}"))
@@ -759,11 +756,11 @@
       ::  try +jump
       ::
       =/  jum  (mule |.((slap old tsgl/[limb/b limb/%jump])))
-      ?:  ?=(%& -.jum)
+      ?:  &((has-arm %jump a old) ?=(%& -.jum))
         =/  via  !<(mark p.jum)
         %-  (trace 4 |.("{<a>} -> {<b>}: via {<via>} per +jump:{(trip a)}"))
         (compose-casts a via b)
-      ?:  ?=(%& -.rab)
+      ?:  &(arm ?=(%& -.rab))
         =/  via  !<(mark p.rab)
         %-  (trace 4 |.("{<a>} -> {<b>}: via {<via>} per +grab:{(trip b)}"))
         (compose-casts a via b)
@@ -787,6 +784,15 @@
       %+  slap
         (with-faces uno+uno dos+dos ~)
       !,(*hoon |=(_+<.uno (dos (uno +<))))
+    ::
+    ++  has-arm
+      |=  [arm=@tas =mark core=vase]
+      ^-  ?
+      =/  rib  (mule |.((slap core [%wing ~[arm]])))
+      ?:  ?=(%| -.rib)  %.n
+      =/  lab  (mule |.((slob mark p.p.rib)))
+      ?:  ?=(%| -.lab)  %.n
+      p.lab
     ::  +build-tube: produce a $tube mark conversion gate from .a to .b
     ::
     ++  build-tube