refactor: Tag class support in registerTag()

src/drop/blank-drop.ts

@@ -1,5 +1,5 @@
-import { isNil, isString, toValue } from '../util/underscore'
-import { EmptyDrop } from '../drop/empty-drop'
+import { isNil, isString, toValue } from '../util'
+import { EmptyDrop } from '../drop'
 
 export class BlankDrop extends EmptyDrop {
   public equals (value: any) {

src/drop/comparable.ts

@@ -1,4 +1,4 @@
-import { isFunction } from '../util/underscore'
+import { isFunction } from '../util'
 
 export interface Comparable {
   equals: (rhs: any) => boolean;

src/drop/empty-drop.ts

@@ -1,6 +1,6 @@
 import { Drop } from './drop'
 import { Comparable } from './comparable'
-import { isObject, isString, isArray, toValue } from '../util/underscore'
+import { isObject, isString, isArray, toValue } from '../util'
 
 export class EmptyDrop extends Drop implements Comparable {
   public equals (value: any) {

src/drop/index.ts

@@ -0,0 +1,7 @@
+export * from './drop'
+export * from './null-drop'
+export * from './empty-drop'
+export * from './blank-drop'
+export * from './forloop-drop'
+export * from './block-drop'
+export * from './comparable'

src/drop/null-drop.ts

@@ -1,6 +1,6 @@
 import { Drop } from './drop'
 import { Comparable } from './comparable'
-import { isNil, toValue } from '../util/underscore'
+import { isNil, toValue } from '../util'
 
 export class NullDrop extends Drop implements Comparable {
   public equals (value: any) {