[Tex/LaTex] Change size and position of subscript (like double subscript)

formattingsubscripts

I'm writing 'cause I would like to know if there's an automatic way (like something to put in the preamble) to make the subscript behave as double subscript: for example if I write

something_a


it should behave as

something_{_a}


You probably don't want to lose one-step subscripts, but to change their behaviour. If you want lower them, you can change TeX parameters as in the following example (the exact values are to be chosen):

\documentclass{article}
\begin{document}

$a_b$

\fontdimen16\textfont2=3pt
\fontdimen17\textfont2=3pt

$a_b \mbox{ 3pt}$

\fontdimen16\textfont2=5pt
\fontdimen17\textfont2=5pt

$a_b \mbox{ 5pt}$

\end{document}


If you also want to change the size of subscripts, you should be more precise: which TeX dialect (e.g., plain or LaTeX) are you interested in?