Function lower 

pub fn lower(
    spec: &ModelSpec,
    numerics: EngineNumerics,
    ctx: usize,
) -> Result<Lowered, ProgramError>

The engine’s step of spec at numerics, read off the decode step the author writes over a cache of ctx positions: the model’s one program, lifted by the shared front end into its step graph and read block by block. The commands do not depend on ctx (the cache’s length is the layout’s); the identity records the cut read.