\setupheadnumber question

Hi,

the wiki says
        If the second argument is signed, the heading’s number will be
        increased or decreased by the specified amount instead.

However, that is not what happens for numbers with a leading '+'.

For example, in

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\starttext
\section{First}
\section{Second}
\section{Third}

\setupheadnumber[section][+10]
\section{Plus 10}

\setupheadnumber[section][-4]
\section{Minus 4}

\setupheadnumber[section][+3]
\section{Plus 3}

\setupheadnumber[section][+1]
\section{Plus 1}
\stoptext
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

The output is (blank lines removed, editorial comments added)

1 First
2 Second
3 Third                      <so far so good>
11 Plus 10                   <huh, '+10' acted like '10'>
8 Minus 4                    <that subtracted 4, then with +1 it is correct>
4 Plus 3                     <nope, should have been 12 (or 11).>
2 Plus 1                     <nope>


Q: is the wiki wrong, or
   is there a bug in the code, or
   am I misinterpreting the wiki sentence?

(I don't think I am misinterpreting the wiki, but I have to admit the
possibility.)

If the code is right and the wiki is wrong, then +N is the same as N, which
seems like a lost opportunity and (arguably) a bit surprising.

Would someone be so kind as to give me a definitive answer?

Thanks.
                                Jim
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