public BaseTerm Eval() // evaluate the term
{
BaseTerm t = ChainEnd();
if (!t.IsEvaluatable)
{
IO.Error("{0} cannot be evaluated by is/2", t);
}
if (t is ValueTerm)
{
return(t); // a ValueTerm stands for itself
}
if (t.IsProperList && !((ListTerm)t).IsEvaluated) // evaluate all members recursively
{
ListTerm result = ListTerm.EMPTYLIST;
List <BaseTerm> tl = ((ListTerm)t).ToList();
for (int i = tl.Count - 1; i >= 0; i--)
{
result = new ListTerm(tl [i].Eval(), result);
}
result.IsEvaluated = true;
return(result);
}
return(t.Apply());
}