Method find_back_until

ppcc/src/parser.cc:86–94

    }
  }
  noinline Int find_back_until(Word64 syms) {
    Int p = pos;
    while (p > marker) {
      p--;
      if (TEST(syms, input->at(p).sym))
        return p;
    }
    return p;
  }
  noinline void safe_skip_until(Word64 syms) {
    // handle parentheses and brackets properly.
    Int par_level = 0;