```python
"""One line of key=value pairs, joined by &."""


def encode_pairs(pairs: list) -> str:
    if not pairs:
        return ""

    def escape(s: str) -> str:
        return s.replace("&", "%26").replace("=", "%3D").replace("%", "%25")

    parts = [f"{escape(key)}={escape(value)}" for key, value in pairs]
    return "&".join(parts)
```