NullFrag          688 third_party/re2/re2/compile.cc   return NullFrag();  // not used by caller
NullFrag          694 third_party/re2/re2/compile.cc       return NullFrag();
NullFrag         1010 third_party/re2/re2/compile.cc   Frag f = c.WalkExponential(sre, NullFrag(), 2*c.max_inst_);
NullFrag         1101 third_party/re2/re2/compile.cc   Frag all = c.WalkExponential(re, NullFrag(), 2*c.max_inst_);