Added contains functions in List and Option. Fixed one type error catch

priv/stdlib/List.aes

@@ -15,10 +15,14 @@ namespace List =
     _::t => Some(t)
 
   function last(l : list('a)) : option('a) = switch(l)
-    []  => None
-    [x] => Some(x)
+    []   => None
+    [x]  => Some(x)
     _::t => last(t)
 
+  function contains(e : 'a, l : list('a)) = switch(l)
+    []   => false
+    h::t => h == e || contains(e, t)
+
 /** Finds first element of `l` fulfilling predicate `p` as `Some` or `None`
  * if no such element exists.
  */

priv/stdlib/Option.aes

@@ -26,6 +26,10 @@ namespace Option =
     None    => abort("Forced None value")
     Some(x) => x
 
+  function contains(e : 'a, o : option('a)) = switch(o)
+    None => false
+    Some(x) => x == e
+
   function on_elem(o : option('a), f : 'a => unit) : unit = match((), f, o)
 
   function map(f : 'a => 'b, o : option('a)) : option('b) = switch(o)