Legacy backend system has been inherited from gEDA gnetlist.
In order lepton-netlist to understand a file is a legacy backend, it must
meet the following requirements:
The name of the backend is defined by removing the prefix gnet-
and the suffix .scm from its file basename. For example, if
backend file name is gnet-custom-backend.scm, its name is
custom-backend and its code has to contain a definition of a
function named custom-backend. The function must take one
argument which is the output file name. Really, it is not necessary
to use this argument in the code as lepton-netlist automatically redirects
the standard output of the function into a file if it is specified by
its command-line arguments.