How is this context free grammar using difference lists in Prolog functioning?

Question

I\'m reading this tutorial on context free grammars in Prolog, and they mention at the bottom of the page implementing a context free grammar in Prolog using difference list

Answer 1

Difference lists work like this ( a layman's explanation).

Consider append is used to join two trains X and Y

X = {1}[2][3][4]     Y = {a}[b][c]

{ } - represents the compartment having the engine or head.

[ ] - represents compartments or elements in the tail. Assume that we can remove the engine from one compartment and put it into another.

Append proceeds like this: The new train Z is now Y i.e., {a}[b][c] next the engine is removed from Z's head and put into tail end compartment removed from X and the new train Z is:

Z = {4}[a][b][c]

The same process is repeated

Z = {3}[4][a][b][c]
Z = {2}[3][4][a][b][c]
Z = {1}[2][[3][4][a][b][c]

our new long train.

Introducing Difference lists is like having a toa hook at the end of X that can readily fasten to Y. The final hook is discarded.

n([man|W],W).

W is the hook here, unification of W with the head of the successor list is the fastening process.