shadow affecting alpha?

[shaggray]

i know this question was posted here a couple of times and i already
knew how to do it, but at the moment i don`t quite get it:

i got a ground plane eith a matte (vraymtlwrapper) material and
want to cast shadows on it and make it affect alpha! i am tweeking
the parameters but there is no shadow in the alpha channel.....?
help needed

[trixian]

Set alpha contribution to -1 (minus one)