https://walkerke.r-universe.dev/spopthttps://walkerke.r-universe.dev/api/packages/spopthttps://walkerke.r-universe.dev/spopt/spopt.pdfhttps://walkerke.r-universe.dev/spopt/doc/manual.htmlhttps://walkerke.r-universe.dev/spopt/NEWShttps://walkerke.r-universe.dev/spopt/NEWS.txthttps://walkerke.r-universe.dev/spopt/citationhttps://walkerke.r-universe.dev/spopt/citation.txthttps://walkerke.r-universe.dev/spopt/citation.cff