scc - detect strongly connected components (logic loops)

yosys> help scc
scc [options] [selection]
This command identifies strongly connected components (aka logic loops) in the
-expect <num>
expect to find exactly <num> SCCs. A different number of SCCs will
produce an error.
-max_depth <num>
limit to loops not longer than the specified number of cells. This
can e.g. be useful in identifying small local loops in a module that
implements one large SCC.
do not count cells that have their output fed back into one of their
inputs as single-cell scc.
Usually this command only considers internal non-memory cells. With
this option set, all cells are considered. For unknown cells all ports
are assumed to be bidirectional 'inout' ports.
-set_attr <name> <value>
set the specified attribute on all cells that are part of a logic
loop. the special token {} in the value is replaced with a unique
identifier for the logic loop.
replace the current selection with a selection of all cells and wires
that are part of a found logic loop
examine specify rules to detect logic loops in whitebox/blackbox cells