import typing


def get_header_from_environ(
    environ: dict, header_name: str
) -> typing.List[str]:
    """Retrieve a HTTP header value from the PEP3333-conforming WSGI environ.

    Returns:
        A list with a single string with the header value if it exists, else an empty list.
    """
    environ_key = "HTTP_" + header_name.upper().replace("-", "_")
    value = environ.get(environ_key)
    if value is not None:
        return [value]
    return []