sentinel
Class that handles sentinel objects that designate the terminus of a part.
Sentinal objects designate the terminus of a part, and exist for all parts (including empty parts).
A sentinal object's brother is the object above the part, and its sister is the object below the part.
When using
shape.has_bro(),
shape.has_sis(), and
shape.has_son(), if the object checked is a sentinel, 'false' is returned.