jenkins conditional matrix - combination filter does not work

[malferns]

I have these axis defined:

platform: a, b, c
mode: m
board_type: x, y


Would like to only build these combinations:
a/x a/y
b/x
c/x

These combos should be excluded:
b/y
c/y

So I created a combination filter (however, this does not work and it builds all the combinations.):
!(platform == "b" && board_type == "y") || !(platform == "c" && board_type == "y")

I finally landed up doing this which works:
(platform=="a") && (board_type=="x") || (platform=="a") && (board_type=="y") || (platform=="b") && (board_type=="x") || (platform=="c") && (board_type=="x")

Would prefer to have the exclusions.  Any thoughts?