Segment.SegsSourceappend friendly segment list
A singleton of the empty segment: [Segment.empty]. Do not confuse with the empty list of segments [].
Append a side to the last segment of t
Append a segment to the last segment of t
Append a list of sides to the last segment of t
Finalize the current last segment of t then append an empty semgent at the last of it
Get the segment list representation of t
The last segment is appendable
The last segment