The grammar will never by itself try to call a subroutine, it only looks for methods in the action object. However, there is a "sum" method that An-Action inherits from Any. You can find it here: https://github.com/rakudo/rakudo/blob/master/src/core/Any.pm6#L440
It's probably a good idea to put an empty method of the same name that accepts one argument (the match object) into the action class. The same problem occurs with any of the other methods that already exist on Any. You can find them like this: perl6 -e 'Any.^methods(:all).>>.name.put'
