on: push
jobs:
  evalm:
    strategy:
      matrix: |-
        ${{fromJson('
          {
            "A": [ "A", "B" ]
          }
        ')}}
    runs-on: ubuntu-latest
    steps:
    - name: Check if the matrix key A exists
      run: |
        echo $MATRIX
        exit ${{matrix.A && '0' || '1'}}
      env:
        MATRIX: ${{toJSON(matrix)}}