class SyntaxTree::ArgParen

def trailing_comma?
  arguments = self.arguments
  return false unless arguments.is_a?(Args)
  parts = arguments.parts
  if parts.last.is_a?(ArgBlock)
    # If the last argument is a block, then we can't put a trailing comma
    # after it without resulting in a syntax error.
    false
  elsif (parts.length == 1) && (part = parts.first) &&
        (part.is_a?(Command) || part.is_a?(CommandCall))
    # If the only argument is a command or command call, then a trailing
    # comma would be parsed as part of that expression instead of on this
    # one, so we don't want to add a trailing comma.
    false
  else
    # Otherwise, we should be okay to add a trailing comma.
    true
  end
end