Restrict some items in Matrix

I’m using the Matrix to build a number of OS/Architecture combinations. However, I don’t want to build every single one. For example, in the following, I want to build all architecture combinations for Xenial, but not for Trusty. I only want amd64 for Trusty. So currently I get 6 build events, when I only want 4 (xenial/all 3 architectures, and trusty:amd64) Is there any easy way to handle this in the drone config?

matrix:
OPERATING_SYSTEM:
- xenial
- trusty

ARCHITECTURES:
- amd64
- arm64
- arm

platform: linux/${ARCHITECTURES}
branches: [develop, master]

pipeline:
build:

Yes, you can enumerate specific matrix combinations [1]

matrix:
  include:
    - OS: xenial
      ARCH: amd64
    - OS: xenial
      ARCH: arm
    - OS: xenial
      ARCH: arm64
    - OS: trusty
      ARCH: amd64

[1] http://docs.drone.io/matrix-builds/

That is perfect. I apologize for missing that in the doc. Thank you, Brad.