singleton - create singleton modules

    singleton [selection]

By default, a module that is instantiated by several other modules is only
kept once in the design. This preserves the original modularity of the design
and reduces the overall size of the design in memory. But it prevents certain
optimizations and other operations on the design. This pass creates singleton
modules for all selected cells. The created modules are marked with the
'singleton' attribute.

This commands only operates on modules that by themself have the 'singleton'
attribute set (the 'top' module is a singleton implicitly).