diff --git a/pkg/zones/zones.go b/pkg/zones/zones.go
index 7601919..a8fb2ea 100644
--- a/pkg/zones/zones.go
+++ b/pkg/zones/zones.go
@@ -66,6 +66,26 @@ func (m Machines) Swap(i, j int) {
 	m[i], m[j] = m[j], m[i]
 }
 
+// FilterMachines produces a subset of the machines offered by the given
+// iterator fulfilling a condition
+func FilterMachines(m MachineIterator, cond func(*Machine) bool) (Machines, int) {
+	var out []*Machine
+
+	if cond == nil {
+		// unconditional
+		cond = func(*Machine) bool { return true }
+	}
+
+	m.ForEachMachine(func(p *Machine) bool {
+		if cond(p) {
+			out = append(out, p)
+		}
+		return false
+	})
+
+	return out, len(out)
+}
+
 // Zone represents one zone in a cluster
 type Zone struct {
 	zones *Zones