class SyntaxTree::Parser

def on_excessed_comma(*)

on_excessed_comma: () -> ExcessedComma
:call-seq:

versions of Ruby it accepted a string literal with a value of ",").
The handler for this event accepts no parameters (though in previous 
def on_excessed_comma(*)
  comma = consume_token(Comma)
  ExcessedComma.new(value: comma.value, location: comma.location)
end