Include explicit disjunction in the union node

The generated parser would be the second place that would construct a
temporary disjunction in the implementation, I think it might as well
contain the disjunction directly.

ebnf.go

@@ -62,7 +62,7 @@ func buildEBNF(root bool, n node, seen map[node]bool, p *ebnfp, outp *[]*ebnfp)
 		p = &ebnfp{name: name}
 		*outp = append(*outp, p)
 		seen[n] = true
-		for i, next := range n.nodeMembers {
+		for i, next := range n.disjunction.nodes {
 			if i > 0 {
 				p.out += " | "
 			}

grammar.go

@@ -30,7 +30,7 @@ func (g *generatorContext) addUnionDefs(defs []unionDef) error {
 		}
 		unionNode := &union{
 			unionDef:    def,
-			nodeMembers: make([]node, 0, len(def.members)),
+			disjunction: disjunction{nodes: make([]node, 0, len(def.members))},
 		}
 		g.typeNodes[def.typ], unionNodes[i] = unionNode, unionNode
 	}
@@ -41,7 +41,7 @@ func (g *generatorContext) addUnionDefs(defs []unionDef) error {
 			if err != nil {
 				return err
 			}
-			unionNode.nodeMembers = append(unionNode.nodeMembers, memberNode)
+			unionNode.disjunction.nodes = append(unionNode.disjunction.nodes, memberNode)
 		}
 	}
 	return nil

nodes.go

@@ -97,16 +97,15 @@ func (c *custom) Parse(ctx *parseContext, parent reflect.Value) (out []reflect.V
 // @@ (for a union)
 type union struct {
 	unionDef
-	nodeMembers []node
+	disjunction disjunction
 }
 
 func (u *union) String() string   { return ebnf(u) }
 func (u *union) GoString() string { return u.typ.Name() }
 
 func (u *union) Parse(ctx *parseContext, parent reflect.Value) (out []reflect.Value, err error) {
 	defer ctx.printTrace(u)()
-	temp := disjunction{u.nodeMembers}
-	vals, err := temp.Parse(ctx, parent)
+	vals, err := u.disjunction.Parse(ctx, parent)
 	if err != nil {
 		return nil, err
 	}

visit.go

@@ -20,7 +20,7 @@ func visit(n node, visitor func(n node, next func() error) error) error {
 		case *custom:
 			return nil
 		case *union:
-			for _, member := range n.nodeMembers {
+			for _, member := range n.disjunction.nodes {
 				if err := visit(member, visitor); err != nil {
 					return err
 				}