Of these two variants the second works.
The first gives as error an old enemy: the \Umathquad error
% gives an error:
\startsetups[test]
$\displaystyle
a=1
$
\stopsetups
\setup[test]
\stoptext
% works:
\starttext
$$\displaylines{a=1\cr b+c+d=3\cr}$$
\stoptext
ConTeXt ver: 2012.01.16 18:33 MKIV
! Math error: parameter \Umathquad\displaystyle is not set.
\8>:test #1->$\displaystyle a=1$
Why this error?
Hans van der Meer