When a geometry is described by a set of axioms, the notion of a line is usually left undefined (a so-called primitive object). The properties of lines are then determined by the axioms which refer to them.
Definition of mull 1: friable forest humus that forms a layer of mixed organic matter and mineral soil and merges gradually into the mineral soil beneath 2: a finely powdered solid especially in a suspension
A pall is the heavy cloth that is draped over a coffin. Hence the metaphoric term casting a pall on a gathering of people, by announcing bad news to the group. By metonymy, the term pallbearer is used to signify someone who bears the coffin which the pall covers.