The Convex dashboard receives two lists of labels and needs one list containing every distinct value from the first list followed by new values from the second list. Preserve the order of each value's first occurrence.
Implement the union without using set, dict, union, append, or other built-in collection methods. You may use indexing, loops, conditionals, len, and list concatenation.
Write union_lists(first, second), where first and second are lists of integers. Return a new list containing each distinct integer exactly once. Values from first appear first in their original order. Values from second are added only when they have not already appeared in the result.
Do not modify either input list.
def union_lists(first, second):