'Yields the anchor tag as parsed from the original token.
Chunks that are anchors have a tag with an "A" prefix (e.g., "A1").
Chunks that are PNP attachmens (or chunks inside a PNP) have "P" (e.g., "P1").
Chunks inside a PNP can be both anchor and attachment (e.g., "P1-A2"),
as in: "clawed/A1 at/P1 mice/P1-A2 in/P2 the/P2 wall/P2"'

@property
def anchor_id(self):

    id = ''
    f = (lambda ch: filter((lambda k: (self.sentence._anchors[k] == ch)), self.sentence._anchors))
    if (self.pnp and self.pnp.anchor):
        id += ('-' + '-'.join(f(self.pnp)))
    if self.anchor:
        id += ('-' + '-'.join(f(self)))
    if self.attachments:
        id += ('-' + '-'.join(f(self)))
    return (id.strip('-') or None)
