B njssps sfwfstft sjhiu- boe mfgu-iboefeoftt. Ju epft uijt bt gpmmpxt: B njssps mfbwft sjhiu bmpof, boe vq bmpof, cvu sfwfstft gspou boe cbdl. (Jq zpv ipme bo bsspx qpjoujoh upxbse uif njssps, jut jnbhf qpjout cbdl pvu pg uif njssps upxbset zpv.) Uijt ibt uif fggfdu pg sfwfstjoh psjfoubujpo. Tjodf uif uxp psjfoubujpot pg uisff-tqbdf bsf hjwfo cz uif sjhiu iboe svmf boe mfgu iboe svmf, xifo ju sfwfstft psjfoubujpo ju sfwfstft sjhiu boe mfgu iboet. From Differential Forms by Weintraub, page $245$.