read_verilog dffs.v
hierarchy -top top
proc
flatten
equiv_opt -assert -map +/ice40/cells_sim.v synth_ice40 # equivalency check
design -load postopt # load the post-opt design (otherwise equiv_opt loads the pre-opt design)
cd top # Constrain all select calls below inside the top module
select -assert-count 1 t:SB_DFF
select -assert-count 1 t:SB_DFFE
select -assert-none t:SB_DFF t:SB_DFFE %% t:* %D