[edit] graphical cycle

• term graphical cycle (HDO_00000021) not yet in hierarchy.
• current def: definition "A non-empty graphical trail in which only the first and the last elements in the sequence of visited nodes refer to the same node.

Suggested changes:

• set as subclass of graphical trail (HDO_00000023)
• adjust def to: definition "A graphical trail which is not empty and in which only the first and the last elements in the sequence of visited nodes refer to the same node.