def METHOD_NAME(package_name, requirements):
    specified = property.select("install-default-prefix", requirements)
    if specified:
        specified = ungrist(specified[0])
    prefix = option.get("prefix", specified)
    requirements = property.change(requirements, "install-default-prefix", None)
    # Or some likely defaults if neither is given.
    if not prefix:
        if os.name == "nt":
            prefix = "C:\\" + package_name
        elif os.name == "posix":
            prefix = "/usr/local"
    return prefix