Boldly Folding (part 1)

Algebra Level 3

You have a rectangular piece of paper of width $W$ and length $L$. You pick up the left side and fold it to the right side, like so:

Which function describes the new position of a point $P = (x, y)$ on the paper after the fold?

$f(x, y, W, L) = \left\{ \begin{array}{rl} (W-x,y) &\mbox{ if $x<\frac{W}{2}$} \\ (x, y) &\mbox{ otherwise} \end{array} \right.$
$f(x, y, W, L) = \left\{ \begin{array}{rl} (W^{2}-x,y+L-W) &\mbox{ if $x>\frac{W}{2}$} \\ (x, y) &\mbox{ otherwise} \end{array} \right.$
$f(x, y, W, L) = (W - x, L-y)$
$f(x, y, W, L) = (\frac{\pi x}{W}, \frac{y}{\pi L})$

Details and assumptions:

Excel in math and science