// without variables
tracematch ()
sym A before: visit( A);
sym B before: visit( B);
regex
//  (# A (* B))

// With variables
// The before bA and after aA must be on the same object a.
// in between is a before visit to a B-object b and the object
// previous to b must be an X-object.
// We want to find all the matches.

tracematch (A a, X x)
sym bA before: (& visit(A) target(a));
sym B before: (& visit(B) this(x));
sym aA after: (& visit(A) target(a));
regex
//  (# bA (* B) aA)