def __init__(self, name, fkey, pmap, nullable=False, unique=True):
self._name = name<EOL>self._fkey = fkey<EOL>self._pmap = pmap<EOL>self._nullable = nullable<EOL>self._unique = unique<EOL>
:param name: Index name.
:type name: str

:param fkey: Function that extracts the indexed value from the indexed table.
:type fkey: callable

:param pmap: Persistent map for index storage.
:type pmap: :class:`zlmdb._pmap.PersistentMap`

:param nullable: Whether the indexed table column is allowed to
    take ``None`` values.
:type nullable: bool

:param unique: Whether the indexed table column must take unique values.
:type unique: bool