make Receive usage stricter, and document

mixing/mixpool/mixpool.go

@@ -651,8 +651,9 @@ PRLoop:
 }
 
 // Received is a parameter for Pool.Receive describing the session and run to
-// receive messages for, and slices for returning results.  Only non-nil slices
-// will be appended to.  Received messages are unsorted.
+// receive messages for, and slices for returning results.  A single non-nil
+// slices is required and indicates which message slice will be will be
+// appended to.  Received messages are unsorted.
 type Received struct {
 	Sid [32]byte
 	Run uint32
@@ -696,6 +697,32 @@ func (p *Pool) Receive(ctx context.Context, expectedMessages int, r *Received) e
 	}
 	rs = &ses.runs[run]
 
+	nonNilSlices := 0
+	if r.KEs != nil {
+		nonNilSlices++
+	}
+	if r.CTs != nil {
+		nonNilSlices++
+	}
+	if r.SRs != nil {
+		nonNilSlices++
+	}
+	if r.DCs != nil {
+		nonNilSlices++
+	}
+	if r.CMs != nil {
+		nonNilSlices++
+	}
+	if r.FPs != nil {
+		nonNilSlices++
+	}
+	if r.RSs != nil {
+		nonNilSlices++
+	}
+	if nonNilSlices != 1 {
+		return fmt.Errorf("mixpool: exactly one Received slice must be non-nil")
+	}
+
 Loop:
 	for {
 		// Pool is locked for reads.  Count if the total number of