Write a function in Calculus λ that returns a list with the path from a to b in a graph

Write a function in Calculus λ that takes as input a directed graph, g, and the identifiers of two vertices, a and b, and returns a list with the path from a to b. If there is no path from a to b your function should return the empty list.

You can represent the graph as you see fit (and explain it) and you can also identify the vertices as you see fit.