Adding intersection to traversal strategies

Suppose we have some traversal strategy S1:
(A . ( B . Z + C . Z)) 

and we are only interested in paths from A to Z which bypass
any edge labeled x. We could write a traversal strategy of the form:
(A . bypassing {-> *,x,*} 
  ( B .bypassing {-> *,x,*} Z + C .bypassing {-> *,x,*} Z)) 

But this strategy contains a lot of duplication.

Instead, we introduce intersection which allows to isolate
the intersection. The above example becomes:

(A . ( B . Z + C . Z)) ^ (A . bypassing {-> *,x,*} Z)

The good news is that Boaz can implement intersection efficiently
by extending the token passing algorithm on the traversal graphs.

do s1 ^ do s2 means to follow only paths which are legal according
to both strategies.

Intersection of strategies has many applications: for example
the hidden edges which Geoff is implementing?!

Another application is that we define some "view" of a class graph
using a strategy s1 and then we define traversals within that view
using a second strategy s2. The first view could be a persistent
object view.

-- Karl