--- a/progs/fun_parser.scala	Sun Oct 13 00:53:20 2019 +0100
+++ b/progs/fun_parser.scala	Mon Oct 14 00:02:24 2019 +0100
@@ -34,13 +34,12 @@
 abstract class Parser[I, T](implicit ev: I => Seq[_]) {
   def parse(ts: I): Set[(T, I)]
 
-  def parse_all(ts: I) : Set[T] =
-    for ((head, tail) <- parse(ts); if (tail.isEmpty)) yield head
-
-  def parse_single(ts: I) : T = parse_all(ts).toList match {
-    case List(t) => t
-    case _ => { println ("Parse Error\n") ; sys.exit(-1) }
-  }
+  def parse_single(ts: I) : T = 
+    parse(ts).partition(_._2.isEmpty) match {
+      case (good, _) if !good.isEmpty => good.head._1
+      case (_, err) => { 
+	println (s"Parse Error\n${err.minBy(_._2.length)}") ; sys.exit(-1) }
+    }
 }
 
 // convenience for writing grammar rules