flatten - flatten design

yosys> help flatten

flatten [options] [selection]
This pass flattens the design by replacing cells by their implementation. This
pass is very similar to the 'techmap' pass. The only difference is that this
pass is using the current design as mapping library.

Cells and/or modules with the 'keep_hierarchy' attribute set will not be
flattened by this command.
Ignore the 'whitebox' attribute on cell implementations.