BGCHK (Function)

is a function which takes one numerical parameter which gets rounded down: a 'layer', 0 for BG foreground layer, 1 for BG background layer (same as the first parameter of the  command). The function will normally return. After a  applied to this layer with an 'interpolation time' of 1 or more, it will return   until the interpolation time expires, but if the interpolation time is 0 or missing, it will return.

If a negative value, or a value of 2 or greater, is passed to the function, the system generates a misleading  error.