Returns the x position of the edge of the screen that is behind the player, in absolute stage coordinates.

Format: BackEdge

Arguments:

- none

Return type: float

Error conditions: none

Notes:

This trigger is equivalent to the expression "ifelse(facing = 1, LeftEdge, RightEdge)".

Example:

`trigger1 = Pos X + CameraPos X < BackEdge
; Triggers if the player is to the left of the back edge of the screen.`