[metapost] What is the best approach for writing LaTeX specific commands in metapost

[Vafa Khalighi]

As an example, when I run mpost -tex=latex vafa.mp, I get this error:

)fatal: Command failed: latex mpf0LcjR.tex; see mpxerr.log
>> mpdefs.mp
>> mpdefs.mpx
! Unable to make mpx file.
l.6 newBox.a(btex
                  apples\strut etex);

but if I delete \selectlanguage{farsi} from vafa.mp and the run mpost
-tex=latex vafa.mp, there is no problem.

Then I come and add the deleted \selectlanguage{farsi} to vafa.mp and run
mpost -tex=latex and strangely this time, there is no problem. Why?


-- 
If some one say: "You divide ten into two parts: multiply the one by itself;
it will be equal to the other taken eighty-one times." Computation: You say,
ten less thing, multiplied by itself, is a hundred plus a square less twenty
things, and this is equal to eighty-one things. Separate the twenty things
from a hundred and a square, and add them to eighty-one. It will then be a
hundred plus a square, which is equal to a hundred and one roots. Halve the
roots; the moiety is fifty and a half. Multiply this by itself, it is two
thousand five hundred and fifty and a quarter. Subtract from this one
hundred; the remainder is two thousand four hundred and fifty and a quarter.
Extract the root from this; it is forty-nine and a half. Subtract this from
the moiety of the roots, which is fifty and a half. There remains one, and
this is one of the two parts.

*Muḥammad ibn Mūsā al-Khwārizmī*
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