Function from_head_slice 

pub fn from_head_slice(
    spec: &HeadSliceSpec,
    names: &StageNames,
    target: &SpatialTarget,
) -> Result<SpatialProgram, SpatialReject>

P = the first|last K of every N of X — group g’s K elements from from land at [g·K, (g+1)·K): one band per group, each an affine read X[c + g·(N − K) + from].