Add the “accepted” token.

It helps to recognize an invalid escape character, such as: Foo\Bar\Baz,
which is equivalent to Foo\\Bar\\Baz.
Add the getVariable() method.

It helps fluid calls, such as:

    $specification->getClause('requires')
                  ->getVariable('i')
                  ->…;

in addition of:

    $specification->getClause('requires')['i']->…;

which is possible since PHP5.4.
Add the interprete() method & praspel() function.