For example, there are packages and their dependencies:

a -> b (a require b)
b -> c & d
x -> a
y -> x
z -> x
z -> x & y

Now, give c and z, there are two dependency paths:

z -> x -> a -> b -> c
z -> y -> x -> a -> b -> c

The question is how to find out the dependency paths between two specific Debian packages?

P.S. The idea behind is to draw the dependency graph for a given set of packages, include the intermediate ones missing in the chain.