Fix course validity failures

Author: zedaizd

example/course-resources/fork/en/images-index.yaml

@@ -12,7 +12,7 @@ asset_index:
           - fork-folded
           - fork-choose-open-repository
           - fork-select-repository-location
-          - fork-open-basic
+          - open-basic
           - folder-history-change
           - graph-fork
           - select-last-commit

example/course-resources/fork/en/level-data/browse-history-index.yaml

@@ -1,9 +1,10 @@
 asset_index:
 # tutorial contents
+  infile:
     introduction: |+2
       .md
       
-      In the end of the previous level, we mentioned that understanding ${{t/graph}} is the key to learning git. The reason is that, as its name suggests, ${{graph}} displays the *revision history* of a git controlled folder graphically.  
+      In the end of the previous level, we mentioned that understanding ${{t/graph}} is the key to learning git. The reason is that, as its name suggests, ${{t/graph}} displays the *revision history* of a git controlled folder graphically.  
       
       *Revision history* sounds pretty abstract, doesn't it? Please refer to the following illustration for what it exactly means.  
 

example/course-resources/fork/en/level-data/intro-open-repository-index.yaml

@@ -77,7 +77,7 @@ asset_index:
       .md
       Congratulations! Now we have successfully opened up a folder using ${{t/gui}}. Now, it should show something like the region marked by the red square in the following image. It is called ${{t/graph-f}}
 
-      ![illustration]({{images/fork-open-basic}})  
+      ![illustration]({{images/open-basic}})  
 
       Please count how many "circles" are displayed on the ${{t/graph}} of your ${{t/gui}}, and fill the answer in the following text box.
     
@@ -88,6 +88,15 @@ asset_index:
       1. How to open ${{t/gui}}
       2. How to open a git controlled folder using ${{t/gui}}
 
-      In the previous block, we mentioned a term "${{graph}}". It is the foundation of how git controls versions of files. Understanding what a ${{t/graph}} is and how it works greatly helps one get familiar with git.  
+      In the previous block, we mentioned a term "${{t/graph}}". It is the foundation of how git controls versions of files. Understanding what a ${{t/graph}} is and how it works greatly helps one get familiar with git.  
       In the following levels, we will have a deeper look into it.  
 
+  fallbacks:
+    default:
+      - path_replacement:
+          - ''
+          - 'zh-Hant'
+        keys:
+          - sound-bird
+          - sound-cicada
+

example/course-resources/fork/en/st-index.yaml

@@ -15,6 +15,7 @@ asset_index:
           - act-stage
           - act-unstage
           - discard-changes
+          - file-tree-gui
           - reset-file-item
           - reset-file-sub-current
           - reset-file-sub-previous

example/course-resources/fork/zh-Hant/level-data/checkout-revision-index.yaml

@@ -175,7 +175,7 @@ asset_index:
       相信你已經非常嫻熟於如何切換${{t/work-tree}}到不同版本上了  
 
       到這裡你或許會開始好奇，在上一個章節中，我們學習了如何瀏覽過往的${{t/file-tree}}  
-      並且學習了可以從${{t/gui}}的${{t/file-tree-gui}}介面中窺探特定檔案在過往版本的內容  
+      並且學習了可以從${{t/gui}}的${{st/file-tree-gui}}介面中窺探特定檔案在過往版本的內容  
       既然如此，我們有需要把資料夾切換回特定的版本嗎？  
 
       實際上是有的。主要有兩種目的  

example/course-resources/fork/zh-Hant/level-data/merge-branch-index.yaml

@@ -112,14 +112,14 @@ asset_index:
       就讓我們來偷看一下在「`${{level-data/merge-branch/branch-name-hamburger-alt-2}}`」這個分支上的「`${{level-data/merge-branch/file-2-name}}`」的內容如何吧  
 
       為了偷看別的${{t/branch}}的檔案內容  
-      我們可以回想前面章節曾經學過的，在${{t/gui}}上操作${{t/file-tree-gui}}介面  
+      我們可以回想前面章節曾經學過的，在${{t/gui}}上操作${{st/file-tree-gui}}介面  
       請仿照下圖(一)操作  
 
       ![illustration]({{images/recall-view-file-tree}})  
       <div class="img-title">圖(一)</div>  
 
       先從${{t/graph}}上圖紅框圈選處所示，選擇目標的${{t/revision}}  
-      然後如上圖橘框圈選處所示，開啟${{t/file-tree-gui}}介面  
+      然後如上圖橘框圈選處所示，開啟${{st/file-tree-gui}}介面  
 
       請檢視「`${{level-data/merge-branch/file-2-name}}`」的內容  
       並回答它的看起來像是下列何者：