Move SubIterator handling out of Base

graph/iterator/all_iterator.go

@@ -103,6 +103,11 @@ func (it *Int64) Next() (graph.Value, bool) {
 	return graph.NextLogOut(it, val, true)
 }
 
+// No sub-iterators.
+func (it *Int64) SubIterators() []graph.Iterator {
+	return nil
+}
+
 // The number of elements in an Int64 is the size of the range.
 // The size is exact.
 func (it *Int64) Size() (int64, bool) {

graph/iterator/fixed_iterator.go

@@ -149,6 +149,11 @@ func (it *Fixed) Next() (graph.Value, bool) {
 	return graph.NextLogOut(it, out, true)
 }
 
+// No sub-iterators.
+func (it *Fixed) SubIterators() []graph.Iterator {
+	return nil
+}
+
 // Optimize() for a Fixed iterator is simple. Returns a Null iterator if it's empty
 // (so that other iterators upstream can treat this as null) or there is no
 // optimization.

graph/iterator/optional_iterator.go

@@ -35,7 +35,7 @@ import (
 	"github.com/google/cayley/graph"
 )
 
-// An optional iterator has the subconstraint iterator we wish to be optional
+// An optional iterator has the sub-constraint iterator we wish to be optional
 // and whether the last check we received was true or false.
 type Optional struct {
 	Base
@@ -95,6 +95,11 @@ func (it *Optional) NextResult() bool {
 	return false
 }
 
+// No subiterators.
+func (it *Optional) SubIterators() []graph.Iterator {
+	return nil
+}
+
 // Check() is the real hack of this iterator. It always returns true, regardless
 // of whether the subiterator matched. But we keep track of whether the subiterator
 // matched for results purposes.

graph/iterator/value_comparison_iterator.go

@@ -159,6 +159,11 @@ func (it *Comparison) NextResult() bool {
 	return true
 }
 
+// No subiterators.
+func (it *Comparison) SubIterators() []graph.Iterator {
+	return nil
+}
+
 func (it *Comparison) Check(val graph.Value) bool {
 	if !it.doComparison(val) {
 		return false