# MetaPost label positioning based on the text baseline

We are all familiar with MetaPost labels:

label.top(btex $a_t$ etex, z0);


The problem with the above is that the positioning is based on the entire label, including the subscript.

How do I calculate the positioning based on the baseline of the text, i.e. only the "a" part, and not the "t" subscript?

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label.top(btex \setbox0=\hbox{$a_t$}\dp0=0pt \box0 etex, z0);