yosys/docs/source/code_examples/selections/select.ys

read_verilog select.v
prep -top test

cd test
select -set cone_a state_a %ci*:-$dff
select -set cone_b state_b %ci*:-$dff
select -set cone_ab @cone_a @cone_b %i
show -prefix select -format dot -notitle \
     -color red @cone_ab -color magenta @cone_a \
     -color blue @cone_b