7   impl (target, types, impls) {  argument
8     if (!impls && !isArray(types)) {
9       impls = types
12     if (!impls && this.isDerivable) {
13       impls = this._defaultImpls
15     if (!impls) {
16       impls = {}
21     checkImpls(this, target, impls)
33       defineMethod(this, name, target, types, impls)
155 function defineMethod (duck, name, target, types, impls) {  argument
182   const fn = impls[name] || duck._defaultImpls[name]
190 function checkImpls (duck, target, impls) {  argument
193       !impls[name] &&
209   Object.keys(impls).forEach(function (name) {