def enforce_vertical_symmetry(pixmap):
    '''Enforces vertical symmetry of the pixelmap.
    Returns a pixelmap with all pixels mirrored in the middle.
    The initial ones still remain.'''
    mirror = []
    for item in pixmap:
        y = item[0]
        x = item[1]

        if x <= IMAGE_APEX:
            diff_x = diff(x, IMAGE_APEX)
            mirror.append((y, x + (2 * diff_x) - 1))

        if x > IMAGE_APEX:
            diff_x = diff(x, IMAGE_APEX)
            mirror.append((y, x - (2 * diff_x) - 1))

    return mirror + pixmap
Enforces vertical symmetry of the pixelmap.
    Returns a pixelmap with all pixels mirrored in the middle.
    The initial ones still remain.