code: public com.google.protobuf.ByteString
        getPlaceholderBytes() {
      java.lang.Object ref = "";
      if (valueCase_ == 9) {
        ref = value_;
      }
      if (ref instanceof java.lang.String) {
        com.google.protobuf.ByteString b = 
            com.google.protobuf.ByteString.copyFromUtf8(
                (java.lang.String) ref);
        if (valueCase_ == 9) {
          value_ = b;
        }
        return b;
      } else {
        return (com.google.protobuf.ByteString) ref;
      }
    }
docs: <pre>
  This is a placeholder only used in nodes defined inside a
  function.  It indicates the attr value will be supplied when
  the function is instantiated.  For example, let us suppose a
  node "N" in function "FN". "N" has an attr "A" with value
  placeholder = "foo". When FN is instantiated with attr "foo"
  set to "bar", the instantiated node N's attr A will have been
  given the value "bar".
  </pre>
  
  <code>optional string placeholder = 9;</code>