nlutmap - map to LUTs of different sizes

    nlutmap [options] [selection]

This pass uses successive calls to 'abc' to map to an architecture. That
provides a small number of differently sized LUTs.

    -luts N_1,N_2,N_3,...
        The number of LUTs with 1, 2, 3, ... inputs that are
        available in the target architecture.

    -assert
        Create an error if not all logic can be mapped

Excess logic that does not fit into the specified LUTs is mapped back
to generic logic gates ($_AND_, etc.).