Draws a horizontal filled box, called rule.
The characteristics of this command can be set by \setupblackrules.
Using this command is far more slower than using the TeX primitives
\vrule, but it save us some tokens.
Code is contained in pack-rul.mkii or pack-rul.mkiv.
\setupcolors[state=start] %not needed in MkIV \starttext \blackrule %default rule \blackrule[color=red, height=1em] %customized rule \stoptext
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