func (st *state) advance(add int) {
	if len(st.str) < add {
		panic("internal error")
	}
	st.str = st.str[add:]
	st.off += add
}
func (st *state) advance(add int) {
	if len(st.str) < add {
		panic("internal error")
	}
	st.str = st.str[add:]
	st.off += add
}
func ( st * state ) advance ( add int ) { if len ( st . str ) < add { panic ( " " ) } st . str = st . str [ add : ] st . off += add }
// advance advances the current string offset.
advance advances the current string offset .