class Treetop::Compiler::PredicateBlock

def compile(index, builder, parent_expression = nil)
  super
  # REVISIT: This is distinctly dodgey, but since we can only be called from
  # two contexts, and it works in both those, I'm going with it for now, as
  # opposed to doing the major refactor of providing a proper way of accessing
  # the parent's accumulator variable.
  p = parent
  p = p.parent while p && !p.respond_to?(:accumulator_var)
  assign_result "lambda #{text_value}.call(#{p ? p.accumulator_var : ""})"
  builder.if_ '!'+result_var do
    builder << "terminal_parse_failure(#{expected})"
  end
end

def expected
  '"<semantic predicate>"'  # Should I include (some of) the text_value here?
end