code: func (st *state) failEarlier(err string, dec int) {
  	if st.off < dec {
  		panic("internal error")
  	}
  	panic(demangleErr{err: err, off: st.off - dec})
  }
docs: // failEarlier is like fail, but decrements the offset to indicate
  // that the point of failure occurred earlier in the string.