Function compile_program_to_spatial_returning 

pub fn compile_program_to_spatial_returning(
    source: &str,
    target: &SpatialTarget,
    results: &BTreeSet<String>,
) -> Result<ProgramLowering, SpatialReject>

compile_program_to_spatial for a program whose RESULTS are declared: results names what outside the program reads — its host, or the next program of a deployment — besides what it shows and the state it carries, and every other tensor only the next statement reads is forwarded (super::passes::forward).