the study of kinship may be abstracted to binary relations between people. for example, if "x" is the parent of "y", the relation may be symbolized as "xpy". the converse relation, that "y" is the child of "x", is written "yp""x". suppose that "z" is another child of "x": "zp""x". then "y" is a sibling of "z" as they share the parent "x": . here the relation of siblings is expressed as the composition "p""p" of the parent relation with its inverse.