Process type-scoped @type against previous context.

Author: davidlehn

lib/context.js

@@ -67,7 +67,7 @@ api.process = ({
   }
 
   // track the previous context
-  const previousContext = activeCtx;
+  const previousContext = activeCtx.previousContext || activeCtx;
 
   // if context is property scoped and there's a previous context, amend it,
   // not the current one

lib/expand.js

@@ -175,7 +175,8 @@ api.expand = ({
         Array.isArray(value) ?
           (value.length > 1 ? value.slice().sort() : value) : [value];
       for(const type of types) {
-        const ctx = _getContextValue(activeCtx, type, '@context');
+        const ctx = _getContextValue(
+          activeCtx.previousContext || activeCtx, type, '@context');
         if(!_isUndefined(ctx)) {
           activeCtx = _processContext({
             activeCtx,
@@ -449,7 +450,8 @@ function _expandObject({
         expandedParent, '@type',
         _asArray(value).map(v =>
           _isString(v) ?
-            _expandIri(activeCtx, v, {base: true, vocab: true}, options) : v),
+            _expandIri(activeCtx.previousContext || activeCtx, v,
+              {base: true, vocab: true}, options) : v),
         {propertyIsArray: options.isFrame});
       continue;
     }