complete previously-writen-to-file inline maths elsewhere

I: anywhere with ```$ [space] [part-of-eq] Ctrl-x Ctrl-o```
II: after ``` \(, \[ or \begin{eq-env}``` with ```[space] [part-of-eq] Ctrl-x Ctrl-o``` in the same line or other lines followed

ATTENTION: 
only try to match form the writen-to-file eqs. 
can only complete eq-env by first typing ```\end{```