[MX1: <SourceX>,<Y>,<W>,<H> , <TargetX> ]
[MX1: <SourceX1,<Y1>,<W1>,<H1>,<TargetX1>,<SourceX2>,<Y2>,<W2>,<H2>,<TargetX2>, … ]

Mirror eXchange

The rectangular field at the position <SourceX>, <Y> of the size <W>, <H> is mirrored and then exchanged with the (identically sized) area at <TargetX>,<Y>.

This modifier is similar to the [MX: ] modifier, with the difference, that [MX: ] supposes the two areas are exactly at mirrored positions, so you do not need to specify the position of the second area.