The problem is in the step combinator definition.

debugger.out

@@ -0,0 +1,115 @@
+[1 [2 [3 4 625 6] 7] 8] -- pass
+[1 [2 [3 4 625 6] 7] 8] ? pass
+[1 [2 [3 4 625 6] 7] 8] && pass
+[1 [2 [3 4 625 6] 7] 8] ++ pass
+[1 [2 [3 4 625 6] 7] 8] || pass
+[1 [2 [3 4 625 6] 7] 8] !- pass
+[1 [2 [3 4 625 6] 7] 8] <{} pass
+[1 [2 [3 4 625 6] 7] 8] <<{} pass
+[1 [2 [3 4 625 6] 7] 8] abs pass
+[1 [2 [3 4 625 6] 7] 8] anamorphism pass
+[1 [2 [3 4 625 6] 7] 8] app1 pass
+[1 [2 [3 4 625 6] 7] 8] app2 pass
+[1 [2 [3 4 625 6] 7] 8] app3 pass
+[1 [2 [3 4 625 6] 7] 8] appN pass
+[1 [2 [3 4 625 6] 7] 8] at pass
+[1 [2 [3 4 625 6] 7] 8] average pass
+[1 [2 [3 4 625 6] 7] 8] b pass
+[1 [2 [3 4 625 6] 7] 8] binary pass
+[1 [2 [3 4 625 6] 7] 8] ccccons pass
+[1 [2 [3 4 625 6] 7] 8] ccons pass
+[1 [2 [3 4 625 6] 7] 8] clear pass
+[1 [2 [3 4 625 6] 7] 8] cleave pass
+[1 [2 [3 4 625 6] 7] 8] clop pass
+[1 [2 [3 4 625 6] 7] 8] codi pass
+[1 [2 [3 4 625 6] 7] 8] codireco pass
+[1 [2 [3 4 625 6] 7] 8] dinfrirst pass
+[1 [2 [3 4 625 6] 7] 8] dipd pass
+[1 [2 [3 4 625 6] 7] 8] disenstacken pass
+[1 [2 [3 4 625 6] 7] 8] down_to_zero pass
+[1 [2 [3 4 625 6] 7] 8] drop pass
+[1 [2 [3 4 625 6] 7] 8] dupd pass
+[1 [2 [3 4 625 6] 7] 8] dupdd pass
+[1 [2 [3 4 625 6] 7] 8] dupdip pass
+[1 [2 [3 4 625 6] 7] 8] dupdipd pass
+[1 [2 [3 4 625 6] 7] 8] enstacken pass
+[1 [2 [3 4 625 6] 7] 8] flatten pass
+[1 [2 [3 4 625 6] 7] 8] fork pass
+[1 [2 [3 4 625 6] 7] 8] fourth pass
+[1 [2 [3 4 625 6] 7] 8] gcd pass
+[1 [2 [3 4 625 6] 7] 8] genrec pass
+[1 [2 [3 4 625 6] 7] 8] grabN pass
+[1 [2 [3 4 625 6] 7] 8] grba pass
+[1 [2 [3 4 625 6] 7] 8] hypot pass
+[1 [2 [3 4 625 6] 7] 8] ifte pass
+[1 [2 [3 4 625 6] 7] 8] ii pass
+[1 [2 [3 4 625 6] 7] 8] infra pass
+[1 [2 [3 4 625 6] 7] 8] infrst pass
+[1 [2 [3 4 625 6] 7] 8] make_generator pass
+[1 [2 [3 4 625 6] 7] 8] mod pass
+[1 [2 [3 4 625 6] 7] 8] neg pass
+[1 [2 [3 4 625 6] 7] 8] not pass
+[1 [2 [3 4 625 6] 7] 8] nulco pass
+[1 [2 [3 4 625 6] 7] 8] nullary pass
+[1 [2 [3 4 625 6] 7] 8] of pass
+[1 [2 [3 4 625 6] 7] 8] pam pass
+[1 [2 [3 4 625 6] 7] 8] pm pass
+[1 [2 [3 4 625 6] 7] 8] popd pass
+[1 [2 [3 4 625 6] 7] 8] popdd pass
+[1 [2 [3 4 625 6] 7] 8] popop pass
+[1 [2 [3 4 625 6] 7] 8] popopop pass
+[1 [2 [3 4 625 6] 7] 8] popopd pass
+[1 [2 [3 4 625 6] 7] 8] popopdd pass
+[1 [2 [3 4 625 6] 7] 8] product pass
+[1 [2 [3 4 625 6] 7] 8] quoted pass
+[1 [2 [3 4 625 6] 7] 8] range pass
+[1 [2 [3 4 625 6] 7] 8] range_to_zero pass
+[1 [2 [3 4 625 6] 7] 8] reco pass
+[1 [2 [3 4 625 6] 7] 8] rest pass
+[1 [2 [3 4 625 6] 7] 8] reverse pass
+[1 [2 [3 4 625 6] 7] 8] roll> pass
+[1 [2 [3 4 625 6] 7] 8] roll< pass
+[1 [2 [3 4 625 6] 7] 8] rollup pass
+[1 [2 [3 4 625 6] 7] 8] rolldown pass
+[1 [2 [3 4 625 6] 7] 8] rrest pass
+[1 [2 [3 4 625 6] 7] 8] run pass
+[1 [2 [3 4 625 6] 7] 8] second pass
+[1 [2 [3 4 625 6] 7] 8] shift pass
+[1 [2 [3 4 625 6] 7] 8] shunt pass
+[1 [2 [3 4 625 6] 7] 8] size pass
+[1 [2 [3 4 625 6] 7] 8] spiral_next pass
+[1 [2 [3 4 625 6] 7] 8] split_at pass
+[1 [2 [3 4 625 6] 7] 8] split_list pass
+[1 [2 [3 4 625 6] 7] 8] sqr pass
+[1 [2 [3 4 625 6] 7] 8] stackd pass
+[1 [2 [3 4 625 6] 7] 8] step_zero pass
+[1 [2 [3 4 625 6] 7] 8] sum pass
+[1 [2 [3 4 625 6] 7] 8] swapd pass
+[1 [2 [3 4 625 6] 7] 8] swons pass
+[1 [2 [3 4 625 6] 7] 8] swoncat pass
+[1 [2 [3 4 625 6] 7] 8] tailrec pass
+[1 [2 [3 4 625 6] 7] 8] take pass
+[1 [2 [3 4 625 6] 7] 8] ternary pass
+[1 [2 [3 4 625 6] 7] 8] third pass
+[1 [2 [3 4 625 6] 7] 8] tuck pass
+[1 [2 [3 4 625 6] 7] 8] unary pass
+[1 [2 [3 4 625 6] 7] 8] uncons pass
+[1 [2 [3 4 625 6] 7] 8] unit pass
+[1 [2 [3 4 625 6] 7] 8] unquoted pass
+[1 [2 [3 4 625 6] 7] 8] unswons pass
+[1 [2 [3 4 625 6] 7] 8] while pass
+[1 [2 [3 4 625 6] 7] 8] x pass
+step failed!
+[1 [2 [3 4 625 6] 7] 8] _step0 pass
+[1 [2 [3 4 625 6] 7] 8] _step1 pass
+[1 [2 [3 4 625 6] 7] 8] _stept pass
+[1 [2 [3 4 625 6] 7] 8] times pass
+[1 [2 [3 4 625 6] 7] 8] _times0 pass
+[1 [2 [3 4 625 6] 7] 8] _times1 pass
+[1 [2 [3 4 625 6] 7] 8] _timest pass
+[1 [2 [3 4 625 6] 7] 8] map pass
+[1 [2 [3 4 625 6] 7] 8] _map? pass
+[1 [2 [3 4 625 6] 7] 8] _mape pass
+[1 [2 [3 4 625 6] 7] 8] _map0 pass
+[1 [2 [3 4 625 6] 7] 8] _map1 pass
+[1 [2 [3 4 625 6] 7] 8] _map2 pass

debugger.py

@@ -0,0 +1,39 @@
+'''
+In order to debug the problem I'm having with definitions breaking a
+zipper expression I need to load a system, load the defs into a dictionary,
+and then run the expression and see if it raises an exception, substituting
+definitions one-at-a-time until I find the one that breaks it.
+'''
+
+from joy.library import default_defs, initialize, inscribe, Def
+from joy.joy import joy
+from joy.parser import text_to_expression
+from joy.utils.pretty_print import trace
+from joy.utils.stack import stack_to_string
+
+
+inscribe(trace)
+dictionary = initialize()
+defs = {}
+default_defs(defs)
+
+
+expression = text_to_expression(
+    '[1 [2 [3 4 25 6] 7] 8]'
+    '[dup mul]'
+    '[dip dip infra dip infra dip infra]'
+    '[[] ccons] step i'
+##    '[[[] ccons] step i]'
+##    'trace'
+    )
+
+for def_name in defs:
+    D = dictionary.copy()
+    D[def_name] = defs[def_name]
+    try:
+        stack, _, d = joy((), expression, D)
+    except:
+        print(def_name, 'failed!')
+    else:
+        print(stack_to_string(stack), def_name, 'pass')
+