actual bugs found:
* backwards_compatibility.lua (undeclared global "helper")
* core.lua (use of undeclared global "helper")
* wml_tags.transform_unit had wrong code to deal with recall_cost
* wrong variable name in cave_map_generator
https://forums.wesnoth.org/viewtopic.php?t=46450 reports a Lua crash if end is given but step is not. The wiki says the default step is 1. Note that if end is negative, the [for] does nothing (per sniity check).