Call
In mathematics graph theory, a single-entry single-exit region in a given graph is an ordered edge pair.
For example, with the ordered edge pair, of distinct control-flow edges a and b where:
where a node x is said to dominate node y in a directed graph if every path from start to y includes x. A node x is said to postdominate a node y if every path from y to end includes x.
So, a and b refer to the entry and exit edge, respectively.