This, of course, produces exactly the same result as the previous illustration.
<context>
\starttext
\midaligned{\bf Abstract}
\startnarrower[2*middle]
This is the document's abstract. It contains enough text for two lines, but no more.
\stopnarrower
\blank[big]
This is the main text of the document. Like the abstract, it contains enough text
for two lines, to show the margins.
\stoptext
</context>