An expression chain is a list of expressions separated by '|'
characters.
While evaluating expressions separated by '|', PHPTAL will stop
its evaluation when an expression value is not null and no error
was raised while evaluating the expression.
As a string: expression is always true, string: always terminates
an expression chain whatever expression may follow.
You can use php: expressions inside expression chains, like any
other expression.
<h1 tal:content="page/title | page/alternativeTitle | php:get_default_title()" />