Fixes #37. Replacing existing priority

autoload/todo/txt.vim

@@ -104,7 +104,7 @@ function! todo#txt#prioritize_add(priority)
 endfunction
 
 function! todo#txt#prioritize_add_action(priority)
-    execute "normal! mq0i(".a:priority.") \<esc>`q"
+    execute 's/^\(([a-zA-Z]) \)\?/(' . a:priority . ') /'
 endfunction
 
 " Modeline {{{1

test/tc_priority.todo.txt

@@ -0,0 +1,7 @@
+# insert_priority
+example task
+# end_insert_priority
+
+# replace_priority
+(A) example task
+# end_replace_priority

test/tc_priority.vim

@@ -0,0 +1,28 @@
+let s:here = expand('<sfile>:p:h')
+let s:context = todo#txt#__context__()
+let s:context['data'] = s:here . '/tc_priority.todo.txt'
+let s:tc = unittest#testcase#new('Priority', s:context)
+
+let s:TODAY = strftime("%Y-%m-%d")
+
+let s:PRIORITY_INSERTED = [
+      \ '(A) example task',
+      \ ]
+
+let s:PRIORITY_REPLACED = [
+      \ '(C) example task',
+      \ ]
+
+function! s:tc.test_insert_priority()
+  call self.data.goto('insert_priority')
+  call todo#txt#prioritize_add('A')
+  call self.assert_equal(s:PRIORITY_INSERTED, self.data.get('insert_priority'))
+endfunction
+
+function! s:tc.test_replace_priority()
+  call self.data.goto('replace_priority')
+  call todo#txt#prioritize_add('C')
+  call self.assert_equal(s:PRIORITY_REPLACED, self.data.get('replace_priority'))
+endfunction
+
+unlet s:tc