x1 is the graph on vertices/nodes x2 (set) and edges x3 (set of ordered or unordered tuples of vertices in x2) and with additional properties x4.
A "web"/"network". An element of x2 is a "place" in a more generic/abstract/metaphoric sense than spatial; must be discrete. Thus, webpages, family diagrams, lattices, pages in a book, cities and roadways on a map, etc. constitute graphs. x1 also includes trees. Notice that both x2 and x3 are sets; an element of x3 is an ordered pair/2-tuple, but the order does not matter unless x1 is oriented, in which case the edge runs from the first node in the tuple to the second node in the tuple. x4 can include defining features, weights, etc. Equivalent to grafu (which is merely the gismu form of this word); very similar to tcana.