[tex-k] spacefactor not turned off after \circb in MFbook

[胡亚捷 (Hu Yajie)]

Page C123, exercise 14.2: change
    Why are the `[...]' and `[...]'  characters
to
    Why are the `[...]' and `[...]' characters

The extra space is due to mfbook.tex (line 7187) saying
    Why are the `\kern1pt{\manual\circa}' and `\kern1pt{\manual\circb}'
where \circb is defined in manmac.tex as
    \chardef\circb=33 % filled quartercircle
and 33 is ASCII code for `!' !

It's also good to search for `{\manual 89:;<=>:}' (slanted METAFONT logo)
in mfbook.tex and turn off the spacefactor produced by the final ``colon''.

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Previous reports:
https://github.com/dine2014/texbook-mfbook-bugs/blob/master/bugs.md