Write a predicate replacefirst$/4$ that substitutes the first argument $x$ with the second argument $Y$ only for the first occurrence of $x$ in a list of elements. If the list is empty the result is the empty list. If $x$ is not in the list, then the result is the list itself.

hint:

The first occurrence can be found when the argument $x$ matches the head of the list $x|T|$

Three cases:

The list is empty, then the result is empty

$x$ matches the head, insert $Y$ in the new list

$x$ does not match the head, replace $x$ with $Y$ in the tail