For object transportation we need the concept of unique backward
path property of a strategy wrt a class graph. 

A strategy SS with source s and target t
has the unique backward path property (ubpp) wrt a class graph
if along each object path allowed by SS
each N(t)-object is contained in a unique N(s)-object.

The ubpp can be checked in the class graph: It is disallowed that there
is a class graph path allowed by SS
from N(s) to N(t) passing through N(s) in between.
In other words, from N(s) via * via N(s) to N(t) is not allowed to define
a path in the class graph.

Examples:
SS=
from c:C via B to A

does not have  unique backward path property:
C = [C] B A.
B = A.

unique:
C = B.
B = A.
A = .

not unique:
C = B.
B = A.
A = [C].

SS1=
from c:C via b:B to a:A
If want to use c and b in a B-object: from C to B must have ubpp
If want to use c and b in a A-object: from C to A must have ubpp and
				      from B to A must have ubpp.