Update README.md

README.md

@@ -87,7 +87,7 @@ The term _property graph_ refers to both a mathematical structure and a data mod
 
 A [_graph_](https://en.wikipedia.org/wiki/Graph_(discrete_mathematics)) is an ordered quintet 𝐺 = (𝑉, 𝐸, 𝐴, _ψₑ_, _ψₐ_) consisting of three pairwise disjoint sets and two functions. 𝑉 is a nonempty set whose elements are called [_vertices_](https://en.wikipedia.org/wiki/Vertex_(graph_theory)) (_nodes_, _dots_, _points_), 𝐸 is a set whose elements are called _undirected edges_ (_undirected links_, _undirected lines_), 𝐴 is a set whose elements are called _directed edges_ (_directed links_, _directed lines_, _arcs_, _arrows_), _ψₑ: E → { {u,v}: u,v ∈ V }_ is a total function mapping each undirected edge to an unordered pair of vertices, and _ψₐ: A → { (u,v): u,v ∈ V }_ is a total function mapping each directed edge to an ordered pair of vertices. An [_undirected graph_](https://en.wikipedia.org/wiki/Graph_(discrete_mathematics)#Graph) is a graph in which 𝐴 ≔ _∅_. A [_directed graph_](https://en.wikipedia.org/wiki/Directed_graph) (_digraph_, _oriented graph_) is a graph in which 𝐸 ≔ _∅_. A [_mixed graph_](https://en.wikipedia.org/wiki/Mixed_graph) is a graph where both directed and undirected edges may exist.
 
-Given undirected edge 𝑒 such that _ψₑ_(𝑒) = {𝑢,𝑣}, we say that 𝑒 is an edge _between_ 𝑢 and 𝑣, 𝑒 _connects_ (_joins_) 𝑢 and 𝑣, and 𝑢 and 𝑣 are _adjacent_. Likewise, given directed edge 𝑎 such that _ψₐ_(𝑎) = (𝑢,𝑣), we say that 𝑒 is an edge _from_ 𝑢 _to_ 𝑣, 𝑎 _connects_ (_joins_) 𝑢 to 𝑣, 𝑣 is _adjacent from_ (_out-adjacent to_) 𝑢, 𝑢 is _adjacent to_ (_in-adjacent to_) 𝑣, 𝑢 is the _tail_ (_source vertex_, _initial vertex_) of 𝑎, and 𝑣 is the _head_ (_target vertex_, _terminal vertex_) of 𝑎. A [_loop_](https://en.wikipedia.org/wiki/Loop_(graph_theory)) is an edge (directed or undirected) connecting a vertex with itself. [_Multiple edges_](https://en.wikipedia.org/wiki/Multiple_edges) (_parallel edges_) are two or more undirected edges connecting the same unordered pair of vertices or directed edges connecting the same ordered pair of vertices. A _simple graph_ is a graph in which multiple edges and loops are not allowed. A [_pseudograph_](http://mathworld.wolfram.com/Pseudograph.html) is a graph in which multiple edges and loops are allowed.
+Given undirected edge 𝑒 such that _ψₑ_(𝑒) = {𝑢,𝑣}, we say that 𝑒 is an edge _between_ 𝑢 and 𝑣, 𝑒 _connects_ (_joins_) 𝑢 and 𝑣, and 𝑢 and 𝑣 are _adjacent_. Likewise, given directed edge 𝑎 such that _ψₐ_(𝑎) = (𝑢,𝑣), we say that 𝑒 is an edge _from_ 𝑢 _to_ 𝑣, 𝑎 _connects_ (_joins_) 𝑢 to 𝑣, 𝑣 is _adjacent from_ (_out-adjacent to_) 𝑢, 𝑢 is _adjacent to_ (_in-adjacent to_) 𝑣, 𝑢 is the _tail_ (_source vertex_, _initial vertex_) of 𝑎, and 𝑣 is the _head_ (_target vertex_, _terminal vertex_) of 𝑎. A [_loop_](https://en.wikipedia.org/wiki/Loop_(graph_theory)) is a directed or undirected edge connecting a vertex with itself. [_Multiple edges_](https://en.wikipedia.org/wiki/Multiple_edges) (_parallel edges_) are two or more undirected edges connecting the same unordered pair of vertices or directed edges connecting the same ordered pair of vertices. A _simple graph_ is a graph in which multiple edges and loops are not allowed. A [_pseudograph_](http://mathworld.wolfram.com/Pseudograph.html) is a graph in which multiple edges and loops are allowed.
 
 An _attributed graph_ is a generic term referring to graphs in which an attribute (_single-attributed graph_) or a set, a bag, or a list of attributes (_multi-attributed graph_) may be associated with each vertex (_vertex-attributed graph_), edge (_edge-attributed graph_), or the graph itself. An _attribute_ may be a nominal value, an ordinal value, a key-value pair, or other annotation. (The term _attributes_ is sometimes used to refer only to key-value pairs, while _labels_ is used to refer to nominal attributes).