Differential geometers do it on smooth contours. Differential geometers do it over and under the curves. Functional analysts do it with compact support. Functional analysts do it with degenerate colonels. Galois theorists do it in a field.
Game theorists do it by dominance.
Game theorists do it with saddle points. Geometers do it with involutions. Geometers do it symmetrically. Graph theorists do it in four colours. Group theorists do it simply.