Add some utility functions to help clarify the merge mode being used …

…when assigning terrains

data/lua/core.lua

@@ -635,6 +635,52 @@ if wesnoth.kernel_type() == "Game Lua Kernel" then
 		if type(side) == 'number' then side = wesnoth.sides[side] end
 		return side.starting_location
 	end
+
+	--[========[Map module]========]
+
+	function wesnoth.map.split_terrain_code(code)
+		return table.unpack(code:split('^', {remove_empty = false}))
+	end
+
+	-- possible terrain string inputs:
+	-- A        A^       A^B      ^        ^B
+	-- implied mode:
+	-- both     base     both     overlay  overlay
+	function wesnoth.map.replace_base(code)
+		local base, overlay = wesnoth.map.split_terrain_code(code)
+		if base == nil then -- ^ or ^B
+			-- There's no base to replace with, so do nothing
+			return ''
+		else
+			-- Use the specified base but ignore the overlay
+			return base .. '^'
+		end
+	end
+	function wesnoth.map.replace_overlay(code)
+		local base, overlay = wesnoth.map.split_terrain_code(code)
+		if overlay == nil or overlay == '' then -- A or A^
+			-- No overlay was specified, so we want to clear the overlay without touching the base
+			return '^'
+		else
+			-- An overlay was specified, so use that and ignore the base
+			new_ter = '^' .. overlay
+		end
+	end
+	function wesnoth.map.replace_both(code)
+		local base, overlay = wesnoth.map.split_terrain_code(code)
+		if base == '' then -- ^ or ^B
+			-- There's no way to find a base to replace with in this case.
+			-- Could use the existing base, but that's not really replacing both, is it?
+			error('replace_both: no base terrain specified')
+		elseif overlay == '' then -- A^
+			-- This would normally mean replace base while preserving overlay,
+			-- but we actually want to replace base and clear overlay.
+			return base
+		else
+			-- It's already going to replace both, so return unchanged
+			return code
+		end
+	end
 else
 	--[========[Backwards compatibility for wml.tovconfig]========]
 	local fake_vconfig_mt = {