formatted lerna completion and used helper function

completion/available/lerna.completion.bash

@@ -1,19 +1,23 @@
 #!/bin/bash
 # Lerna autocompletion.
-function _lerna_completions() {
-  local cur compls
 
-  # The currently-being-completed word.
-  cur="${COMP_WORDS[COMP_CWORD]}"
+if _command_exists lerna
+then
+    function _lerna_completions() {
+        local cur compls
 
-  # Options
-  compls="add bootstrap changed clean create diff exec \
-          import init link list publish run version    \
-           --loglevel --concurrency --reject-cycles    \
-           --progress --sort --no-sort --help          \
-           --version"
+        # The currently-being-completed word.
+        cur="${COMP_WORDS[COMP_CWORD]}"
 
-  # Tell complete what stuff to show.
-  COMPREPLY=($(compgen -W "$compls" -- "$cur"))
-}
-complete -o default -F _lerna_completions lerna
+        # Options
+        compls="add bootstrap changed clean create diff exec \
+        import init link list publish run version    \
+        --loglevel --concurrency --reject-cycles    \
+        --progress --sort --no-sort --help          \
+        --version"
+
+        # Tell complete what stuff to show.
+        COMPREPLY=($(compgen -W "$compls" -- "$cur"))
+    }
+    complete -o default -F _lerna_completions lerna
+fi