removeFirst(n, [ ]) = [ ]

Example: removeFirst(3, [ ]) = [ ].

removeFirst(n, L) = tail(L)  when head(L) = n.

Example: removeFirst(2, [2,4,6]) = [4,6].

removeFirst(n, L) = [head(L)] ++ removeFirst(n, tail(L))

Example: removeFirst(6, [2,4,6,8,10]) = [2] ++ removeFirst(6, [4,6,8,10]) = [2] ++ [4,8,10] = [2,4,8,10].