Free variable analysis.

src/syntax/parser.lalrpop

@@ -150,7 +150,7 @@ TypeName: Name = {
 // to run through a few examples. Consider thinking about how you want to
 // parse something like "1 + 2 * 3", for example, versus "1 + 2 + 3" or
 // "1 * 2 + 3", and hopefully that'll help.
-Expression: Expression = {
+pub Expression: Expression = {
     BindingExpression,
 }
 
@@ -271,4 +271,4 @@ Comma<T>: Vec<T> = {
             v
         }
     }
-};
+};