The appropriate use of semi colons after a handler is not clear.  The docs say:

http://undertow.io/undertow-docs/undertow-docs-2.0.0/#textual-representation-of-handlers

Handlers are executed in order and separated by line breaks or semi colons.

This implies that a semicolon should always be valid after a handler.  The semi colon is certainly optional as shown after the first handler here: 

path(/foo) -> response-code(404) else response-code(503);

 And a semi colon is allowed when the first handler is encased in curly braces: 

path(/foo) -> { response-code(404); } else { response-code(503); }

However, this example throws a parsing error: 

path(/foo) -> response-code(404); else response-code(503);

With the error: 

Error parsing predicated handler string Invalid expression

I think that last example should be valid since the next text after the first handler is "else".  I've been caught off guard a few times when I put semi colons after handlers, but then I have an example like above and receive a parsing error.  It doesn't seem necessary to need to use the curly braces unless I have more than one handler I want to group together. 

The predicate DSL is clearly borrowing from most scripting languages like Java or JS where pseudo code like this would be valid: 

if( condition )
  doSomething();
else
  doElse();

In that pseudo code, the first semi colon doesn't terminate the entire if statement, so it doesn't seem correct that the Undertow Predicate language treats it that way.