Consider this definition of member$/2,$ which is true when a list contains a particular element.

member $(x,[x]).$

member$(x,{[}_{|}L])$:$-$member$(x,L).$

Consider the two queries below. Why do they have different behavior?

$?-L=[1,2,3,4],$ member $(3,L).$

$?-$ member $(3,L),L=[1,2,3,4].$

In the first query, member $(3,$ L$)$ will find infinitely many lists containing $3,$ but all except for one will be rejected

In the second query, member $(3,$ L$)$ will find infinitely many lists containing $3,$ but all except for one will be rejected

These queries will have the same behavior

Both queries have no defined behavior, and all things are possible