command: implement node structure

Signed-off-by: Ophestra <cat@gensokyo.uk>

command/node.go

@@ -0,0 +1,36 @@
+package command
+
+import (
+	"flag"
+	"io"
+)
+
+type node struct {
+	child, next *node
+	name, usage string
+
+	out  io.Writer
+	logf LogFunc
+
+	f   HandlerFunc
+	set *flag.FlagSet
+}
+
+func (n *node) adopt(v *node) bool {
+	if n.child != nil {
+		return n.child.append(v)
+	}
+	n.child = v
+	return true
+}
+
+func (n *node) append(v *node) bool {
+	if n.name == v.name {
+		return false
+	}
+	if n.next != nil {
+		return n.next.append(v)
+	}
+	n.next = v
+	return true
+}