Properly indent after =>.

I'm being lazy and not investigating why it doesn't match
the word boundary...

lua/lean/indent.lua

@@ -1,19 +1,19 @@
 local M = {}
 
 -- Borrowed from the (not merged) https://github.com/leanprover/vscode-lean4/pull/329/
-local INDENT_AFTER = vim.regex(([[\<\(%s\)$]]):format(table.concat({
-  'by',
-  'do',
-  'try',
-  'finally',
-  'then',
-  'else',
-  'where',
-  'from',
-  'extends',
-  'deriving',
+local INDENT_AFTER = vim.regex(([[\(%s\)$]]):format(table.concat({
+  '\\<by',
+  '\\<do',
+  '\\<try',
+  '\\<finally',
+  '\\<then',
+  '\\<else',
+  '\\<where',
+  '\\<from',
+  '\\<extends',
+  '\\<deriving',
+  '\\<:=',
   '=>',
-  ':=',
 }, [[\|]])))
 local NEVER_INDENT = vim.regex(([[^\s*\(%s\)]]):format(table.concat({
   'attribute ',

spec/indent_spec.lua

@@ -68,6 +68,26 @@ describe('indent', function()
     )
   )
 
+  it(
+    'indents after =>',
+    helpers.clean_buffer(
+      [[
+        example {n : Nat} : n = n := by
+          induction n with
+          | zero =>
+      ]],
+      function()
+        helpers.feed 'Gorfl'
+        assert.contents.are [[
+          example {n : Nat} : n = n := by
+            induction n with
+            | zero =>
+              rfl
+      ]]
+      end
+    )
+  )
+
   it(
     'respects shiftwidth',
     helpers.clean_buffer([[structure foo where]], function()