added missing file

Author: Numero7

README.md

@@ -13,7 +13,7 @@ and other coordinates are ω (see Algorithm 1 in the paper for further details).
 The fixpoint computation makes use of a subprocedure: an algorithm solving the so-called `path problem`.
 Given a symbolic arena and a (finite) set of abstract configurations F in the arena,
 this problem asks whether, from every finite configuration of the arena,
-there exists a path within the arena that reaches a configuration in (the ideal generated by) F.
+there exists a path within the arena that reaches a configuration in (the downward-closed set generated by) F.
 This is solved by computing a semigroup called the symbolic flow semigroup (see Theorem 26 in the paper for further details).
 
 ## Quick start