Base nexting on interface satisfaction

This is done unsubtlely at the moment and there is plenty of room for
optimisation of assertion location to prevent repeated reasserting as is
done now.

graph/iterator.go

@@ -14,8 +14,7 @@
 
 package graph
 
-// Define the general iterator interface, as well as the Base iterator which all
-// iterators can "inherit" from to get default iterator functionality.
+// Define the general iterator interface.
 
 import (
 	"strings"
@@ -86,19 +85,10 @@ type Iterator interface {
 	// All of them should set iterator.Last to be the last returned value, to
 	// make results work.
 	//
-	// Next() advances the iterator and returns the next valid result. Returns
-	// (<value>, true) or (nil, false)
-	Next() (Value, bool)
-
 	// NextResult() advances iterators that may have more than one valid result,
 	// from the bottom up.
 	NextResult() bool
 
-	// Return whether this iterator is reliably nextable. Most iterators are.
-	// However, some iterators, like "not" are, by definition, the whole database
-	// except themselves. Next() on these is unproductive, if impossible.
-	CanNext() bool
-
 	// Check(), given a value, returns whether or not that value is within the set
 	// held by this iterator.
 	Check(Value) bool
@@ -145,6 +135,25 @@ type Iterator interface {
 	UID() uint64
 }
 
+type Nexter interface {
+	// Next() advances the iterator and returns the next valid result. Returns
+	// (<value>, true) or (nil, false)
+	Next() (Value, bool)
+
+	Iterator
+}
+
+// Next is a convenience function that conditionally calls the Next method
+// of an Iterator if it is a Nexter. If the Iterator is not a Nexter, Next
+// return a nil Value and false.
+func Next(it Iterator) (Value, bool) {
+	if n, ok := it.(Nexter); ok {
+		return n.Next()
+	}
+	glog.Errorln("Nexting an un-nextable iterator")
+	return nil, false
+}
+
 // FixedIterator wraps iterators that are modifiable by addition of fixed value sets.
 type FixedIterator interface {
 	Iterator