Explain an unfamiliar function through its data flow

Explain this code for A developer comfortable with Python syntax but new to generators.:
def unique(items):
    seen = set()
    for item in items:
        if item not in seen:
            seen.add(item)
            yield item

Focus question: Does this preserve input order, and what kinds of inputs can fail?

Start with a plain-language statement of what the code does. Map inputs, key transformations, branches, returned values, and visible side effects. Refer to actual symbols or line numbers from the supplied snippet; do not invent surrounding modules.

Walk through one small example by showing the value changes that can be derived from the code. Label this a manual trace, not an executed result. Explain one relevant boundary or failure case, and identify behavior that depends on a caller, dependency, or configuration not provided.

Finish by answering the focus question and naming the smallest additional excerpt needed to resolve any uncertainty. Do not rewrite the implementation unless asked.