Trees can be created with this model.
There is a class, which represents the nodes of a tree.
A link of the association Parentship can be created
between two nodes.
A parent may have several children
but only one parent or no parent,
if it is the root node.

Tree.use:
one class TreeNode
one association Parantship
one invariant (does not allow cycles)
four complex operations:
  - childPlus1 (recursive) returns all objects
    the calling object is connected with (transitive).
    The union of sets, collect and flatten is used.
    Does not terminate if there are cycles in the graph.
  - childPlus2 like childPlus2; it uses the operation
    childPlusOnNodeSet
  - childPlusOnNodeSet (recursive) collects all
    objects the objects in the set (argument)
    are connected with (transitive) and checks for all
    elements if all directly connected children are
    also element of the set (termination condition).
    With this check, cycles are no problem.
  - childPlus3 can handle cycles
    because the search depth equals the maximum depth
    of the tree.

Tree.cmd:
creates a graph with cycles

Tree2.cmd:
creates a graph with two nodes
the invariant evaluates to false