/// <summary>
/// The walk operator searches for a variable's value in the substitution
/// </summary>
private static object Walk(object u, Substitution s)
{
// When a non-variable term is walked, the term itself is returned
var uvar = u as LogicVariable;
if (uvar == null || !s.ContainsKey(uvar))
{
return(u);
}
return(Walk(s[uvar], s));
}