using Symata; isymata()

C = [a,[[a1,[a12,b12,c12]],[b2,[a22,b22,c22]],[c3,[a32,b32,c32,d32]]]];

begin
trav(tree_List) := Flatten(trav([], tree), 1)
trav(accum_List, [x_, y_List]) := Map(yy -> trav([accum, x], yy), y)
trav(x_,y_) := Flatten([x,y])
end

trav(C)