match dff
    // @TODO: Support more DFF types
    select dff->type == $_DFF_P_
endmatch

match input
    select input->attributes.count(\nan_f2b)
    index <SigSpec> port(input, \Y) === port(dff, \D)
    optional
endmatch

match output
    select output->attributes.count(\nan_b2f)
    index <SigSpec> port(output, \A) === port(dff, \Q)
    optional
endmatch

if (!input and !output)
    reject;