Talk:Mayer–Vietoris sequence/to do


 * Lead: lead image; repetition of "holds" (x2); sentences starting with "The" (x4)
 * In "Background, motivation and history": more citations, "... like the fundamental group, this is an important topological invariant ..."; picture of Mayer
 * Other: functors; more advanced examples; picture in "Further discussion"; clear function
 * Provide a category-theoretic treatment.