x1 is a generalized cylinder (see notes for details) of form/shape x2.
x1 is a set of points/shape (of any number of dimensions) such that x1 is connected and there exists a line L such that for any line L' that is parallel to L, if L' has nonempty intersection with x1, then there exists exactly one subinterval (line segment, ray, or line) J of L' such that J has nonzero (Lebesgue) measure and any subset of L' which nonemptily intersects x1 is contained in J. An axially-perpendicular cross-section of x1 probably is necessary to specify x2. See also: slanu, fru'austumu.