procedure foldr(f, accumulated, list)
    if *list = 0 then
	    return accumulated
	else
        return f(list[1], foldr(f, accumulated, list[2:0]))
end

procedure foldl(f, accumulated, list)
    if *list = 0 then
	    return accumulated
	else
        return foldl(f, f(accumulated, list[1]), list[2:0])
end

procedure add(a, b)
    return a + b
end

procedure concat(value, accumulated)
    return value || accumulated
end

procedure eo_helper(thusFar, elem)
    if elem % 2 == 0 then
	    return [thusFar[1]+1, thusFar[2]]
	else
	    return [thusFar[1], thusFar[2]+1]
end

procedure paired(s)
	return foldl(paired_helper, 0, s) == 0
end
    
procedure paired_helper(count, c)
    if count = -1 then
	    return -1
		
    case c of {
	    "(": return count + 1
	    ")": return count - 1
        default: return count
        }
end

procedure show(s)
    band := repl("-",5) || "   "
    write("\n", band, s, reverse(band))
end

procedure main()
    &trace := -1
	
	show("foldr(add, 0, [1,2,3,4,5])")
	foldr(add, 0, [1,2,3,4,5])
	
	show("foldl(add, 0, [1,2,3,4,5])")
	foldl(add, 0, [1,2,3,4,5])
	
	show("foldr(concat, \"\", [1,2,3,4,5])")
	foldr(concat, "", [1,2,3,4,5])

	show("foldl(\"||\", \"\", [1,2,3,4,5])")
	foldl("||", "", [1,2,3,4,5])

	show("paired(\"(())()\")")
	paired("(())()")
	
	show("paired(\"(()\")")
	paired("(()")

	show("eo_helper, [0,0], [1,3,5,6,7])")
	foldl(eo_helper, [0,0], [1,3,5,6,7])
end