Add a few tla symbols and load them to prettify them on command

- define some TLA+ symbols
- Use `M-x prettify-symbols-mode' to view them.

lisp/tla+-mode.el

@@ -1267,5 +1267,44 @@ The procedure works by:
   (use-local-map widget-keymap)
   (widget-setup)))
 
+
+(defun tla+/load-symbols ()
+  "make some word or string show as pretty Unicode symbols"
+  (setq prettify-symbols-alist
+        '(
+	  ("<"   . ?<)
+	  (">"   . ?>)
+	  ("<>"  . ?◇)
+	  ("<="  . ?≤)
+	  ("\\leq"  . ?≤)
+	  ("\\geq"  . ?≥)
+	  (">="  . ?≥)
+	  ("~>" . ?⇝)
+	  ("\\E" . ?∃)
+	  ("\\A" . ?∀)
+	  ("\\cup" . ?∪)
+	  ("\\union" . ?∪)
+	  ("\\cap" . ?∩)
+	  ("\\intersect" . ?∩)
+	  ("\\in"    . ?∈)
+	  ("\\notin" . ?∉)
+	  ("#" . ?≠)
+	  ("/=" . ?≠)
+	  ("<<" . ?⟨ )
+	   (">>" . ?⟩ )
+	  ("[]" . ?□)
+	  ("\\equiv" . ?≡)
+	  ("<=>"    . ?≡)
+	  ("/\\"    . ?∧)
+	  ("\\/"     . ?∨)
+          ("=>"     . ?⇒)    ; ⇒
+	  ("=="     . ?≜)
+	  ("\\neg"   . ?¬)
+	  ("\\lnot"  . ?¬)
+	  ("\\neg"   . ?¬)
+          )))
+
+(add-hook 'tla+-mode-hook 'tla+/load-symbols)
+
 
 (provide 'tla+-mode)